Arxiu d'etiquetes: evolution

From traditional medicine to personalized medicine

From prehistory, where medicine started began with plants, minerals and parts of animals; until today, medicine has evolved very quickly. Much of the “fault” of his fact is due to genetics, which allows us to talk about personalized medicine. In the following article we discuss this.

THE EVOLUTION OF DISEASES

To talk about medicine, we have first to know diseases. We cannot think that all diseases are genetic, but there are diseases related to anatomical changes, fruit of our evolution.

Chimpanzees are the closest animal to us, humans, with which we share 99% of our genome. Despite this, humans have very particular phenotypic characteristics as the brain most develop, both in size and expansion of the cerebral cortex; hairless sweaty skin, bipedal posture and prolonged dependence on offspring, allowing the transmission of knowledge for longer; among other.

Possibly, the bipedal position was key to the early development of the divergence between the chimpanzee lineage and that of humans; and is also the reason for the appearance of some diseases related to anatomical factors. Among them are hernias, haemorrhoids, varices, disorders of the spine, such as herniated intervertebral discs; osteoarthritis in the knee joint, uterine prolapse and difficulties in childbirth.

The fact that the pelvis was remodelled (Figure 1) and narrower resulted in obstetric problems millions of years later, when the brain expanded. Consequently, the skull as well. The heads of the foetuses were longer and larger, making birth difficult. This explains why the deliveries of humans are longer and longer compared to those of chimpanzees and other animals.

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Figure 1. Comparison between human pelvis and chimpanzee pelvis in bipedal position (Source: Libros maravillosos – La especie elegida (capítulo 5))

The evolution towards modern life has behaved many changes in every way. In comparison to our hunter-gatherer ancestors (Figure 2), our diet has changed a lot and has nothing to do with what other primates eat. For the latter, the fruit represents most of the intake, but for us it is red meat. In addition, we are the only animals that continue to feed us milk after the lactation period.

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Figure 2. Picture of hunter-gatherer humans (Source: Río Verde en la historia

If we add to the sedentary lifestyle and the limited physical activity of modern humans, it can help explain the seriousness and frequency of some modern human diseases.

Lifestyle can also affect us. For example, myopia, which rate is higher in western individuals who read a lot or do activities of near vision, compared to individuals of Aboriginal’s towns.

Another clear example is the alteration in the female reproductive stage. Currently, women have children more and more later. This is also linked to a decrease in the duration of breastfeeding. These changes, which can be considered socially positive, have negative effects on the health of the reproductive organs. It has been shown that the combination of early menarche, limited or no breastfeeding and later menopause are the main risk factors for breast and ovarian cancer.

Humans increasingly live more years and we want the best quality of life. It is easy for more longevity to appear more diseases, by the deterioration of the organism and its cells.

THE EVOLUTION OF MEDICINE

The history of medicine is the history of the struggle of men against disease and since the beginning of this century, is also the history of human effort to maintain health.

We have acquired the scientific knowledge of medicine based on observation and experience, but it has not always been so. Our ancestors experienced sickness and the fear of death before a rational picture could be made of them, and the medicine of that time was immersed in a system of beliefs, myths and rites.

However, in the last years it has been born personalized genomics, which tells you your risk factors. This opens a door to personalized medicine, which adjusts treatments to patients depending on their genome (Figure 3). It uses information from a person’s genes and proteins to prevent, diagnose and treat a disease, all thanks to the sequencing of the human genome.

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Figure 3. Personalized medicine that treats people individually, according to their genome (Source: Indiana Institute of Personalized Medicine)

Molecular methods that make precision medicine possible include tests of gene variation, proteins, and new treatments targeting molecular mechanisms. With the results of these tests and treatments can determine the state of the disease, predict the future state of the disease, the response to the drug and treatment or even the role of the food we eat at certain times, which results of great help to the doctors to individualize the treatment of each patient.

To do this, we have within our reach the nutrigenetics and the nutrigenomics, that like the pharmacogenetics and the pharmacogenomics, they help the advance of a medicine is more and more directed. Therefore, these disciplines are today one of the pillars of personalized medicine since it involves treating each patient individually and tailor-made.

The evolution towards precision medicine is personalized, preventive, predictive and participatory. There is increasing access to information and the patient is more proactive, getting ahead of problems, preventing them or being prepared to deal with them efficiently.

REFERENCES

  • Varki, A. Nothing in medicine makes sense, except in the light of evolution. J Mol Med (2012) 90:481–494
  • Nesse, R. and Williams, C. Evolution and the origins of disease. Sci Am. (1998) 279(5):86-93
  • Mackenbach, J. The origins of human disease: a short story on “where diseases come from”. J Epidemiol Community Health. (2006) 60(1): 81–86
  • Main picture: Todos Somos Uno

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Cutting up dinosaur’s evolutionary tree

For more than 130 years dinosaurs have been classified into two distinct orders, the saurischians and the ornithischians. But as it is common in biological sciences, every theory is true until the opposite is proved. A new study has called into question classical dinosaur classification, destroying and redistributing some of the different dinosaur groups. Even if this new hypothesis isn’t 100% sure yet, in this entry we’ll explain what this dinosaur reordering consists in.

TRADITIONAL DINOSAUR CLASSIFICATION

Since the XIX century, dinosaurs have been divided into two large orders based on their hip anatomy. The order Saurischia (lizard-hipped) includes theropods (carnivorous dinosaurs and current birds) and sauropodomorphs (large, long-necked herbivores); the order Ornithischia (bird-hipped) includes ornithopods (herbivorous and duck-billed dinosaurs), marginocephalians (dinosaurs with horns and hardened skulls) and thyreophorans (armored dinosaurs).

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Traditional dinosaur evolutionary tree by Zureks, with the two different hip morphologies at the bottom.

Yet, this classification doesn’t have the last word. Palaeontology is an extremely volatile science, as with each new discovery, you can dismantle everything you knew at that moment, even if it’s a centenary-old hypothesis. This is what has recently happened with dinosaurs.

THE RISE OF A NEW HYPOTHESIS

A new study published in March 2017, has caused the reconsideration of traditional dinosaur classification. Many previous studies assumed the Saurichia/Ornithischia classification as true and so, the used characters and taxons were all focussed on this classification. However, this new study has pioneered in many aspects:

  • It includes a larger number of species and taxons (many more than in previous investigations).
  • Previous studies gave more importance to basal theropod and sauropodomorph dinosaurs (traditional saurischians), as they were the first dinosaurs to diversify, including few basal ornithischians.
  • It has also included many dinosauromorph archosaurs (non-dinosaur taxons).
  • Older studies had assumed many ornithischian characters to be symplesiomorphies (ancestral characters of all dinosaurs) and they only focused on a few synapomorphies (characters found in a monophyletic group).

This study has detached from many of the previous assumptions on dinosaur phylogeny and has analysed a large number of species and many characters not included in previous investigations. This has made the resulting evolutionary tree pretty different from the ones obtained before.

RESHAPING THE TREE

Then, how does the dinosaur’s evolutionary tree stand according to this hypothesis? Well, the matter is somewhat complex, even if the different groups are still divided in two orders:

  • Order Saurischia which, according to this study, only includes sauropodomorphs and herrerasaurids (a group of carnivorous, non-theropod saurischians).
  • The new order Ornithoscelida (bird-limbed) that includes the traditional ornithischians and theropods, which are no longer saurischians.

Keeping this in mind, let’s now see the characteristics that define these two orders.

Saurischians

The order Saurischia is almost the same, except that theropods are no longer part of this group. This order presents the original saurischian hip structure, which the dinosaurs’ ancestors also had. According to this new hypothesis,  herrerasaurids and sauropodomorphs are all included as saurischians.

Herrerasaurids (Herrerasauridae family) were a small group of basal saurischians that evolved towards meat-eating. That’s why for a long time it was thought that they were the sister-taxon of theropods, but it was later seen that they were found among the first saurischians. Even if they were pretty specialized, they were probably displaced by competition with other predators, appearing during the middle Triassic and becoming extinct at the end of it.

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Photo by Brian Smith of a Herrerasaurus skeleton and model, from the Field Museum of Natural History of Chicago.

Herrerasaurids occupied a similar ecological niche as theropods. The new hypothesis implies that hypercarnivory (feeding exclusively on meat) evolved independently twice in dinosaurs, which makes some palaeontologist question it. Yet the herrerasaurid and theropod anatomy differed in some aspects, such as the anatomy of their hands (more generalistic in herrerasaurids) and the jaw structure.

The first sauropodomorphs were biped animals just like herrerasaurids, even if they were omnivorous. Yet, sauropodomorphs would end up becoming huge herbivorous quadrupeds with characteristic long necks.

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Thecodontosaurus skeleton (by Qilong), a basal sauropodomorph and a reconstruction of Plateosaurus (from Walters, Senter & Robins) a more advanced one. Even if it cannot be appreciated in this image, sauropodomorphs would increase very their size very much during their evolution (Thecodontosaurus 2 metres, Plateosaurus up to 10 metres).

Ornithoscelidans

The new dinosaur order is Ornithoscelida, which groups theropods with ornithischians. This taxon is supported by more than twenty skeletal synapomorphies (derived characters shared by a clade), present both in basal theropods and ornithischians. Some of these characteristics include the presence of a gap between premaxillar and maxillar teeth (diastema) and the fusion of the ends of the tibia and the fibula into a tibiotarsus (even if these characteristics are only found on the most basal species).

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Scheme from Baron et al. (2017) of the skulls of two basal ornithoscelidans, Eoraptor (a theropod, top) and Heterodontosaurus (an ornithischian, bottom).

Both theropods and the first ornithischians were bipedal animals. Also, the presence of heterodont teeth in the ancestral members of both groups leads us to think that the first ornithoscelidans were omnivorous, which would later specialise in feeding on meat and on plants (theropods and ornithopods respectively).

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Reconstruction of the face of Daemonosaurus, one of the first theropods, by DeadMonkey8984.

A curiosity about the new classification is that accepting Ornithoscelida as a valid taxon, all feathered dinosaurs are put together into one group. Everyone knows that many theropods presented feathers (as they were the ancestors of birds) but, what most people don’t know is that feathers have also been found in some basal ornithischians and in more advanced ones too.

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Reconstruction by Tom Parker of Kulindadromeus, a ornithischian which feathers have been proved to be present on most of its body.

KEEP INVESTIGATING

Then, is this hypothesis irrevocable? Well, no of course. Even if it’s pretty tempting to assume that the dinosaur’s natural history has been changed, we cannot say that from now on dinosaurs will be classified this way.

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Dinosaur evolutionary tree according to Baron et al. (2017), in which we can see the different clades; Dinosauria (A), Saurischia (B) and Ornithoscelida (C).

Even if this study shows really interesting results about the origin of dinosaurs, we cannot dismiss hundreds of previous studies about this group of animals. We’ll have to remain alert to new articles that step by step will keep unveiling more information about the relationships between these Mesozoic reptiles. And that’s what’s so stimulating about biology, that there’s nothing sure! And that with new investigation techniques and new discoveries, little by little we learn more about the world around us.

Keep your mind open and keep investigating!

REFERENCES

The following sources have been consulted during the elaboration of this entry:

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Metal hyperaccumulation in plants

During million years the evolution leaded plants to develop different strategies to defence from natural enemies, giving rise to an evolutionary weaponry war in which the survival of ones and others depends into the ability to beat the other’s adaptations. It is in that scenario where the high-level accumulation of heavy metals in plants plays an important role.

INTRODUCTION

Boyd (2012) commented that plant defences can be grouped in different categories:

  • mechanic: thorns, coverage, etc.
  • chemical: different organic and inorganic components.
  • visual: crypsis and mimicry .
  • behavioural: related with phenology’s modification.
  • and associative: symbiosis with other organisms, such is the case of the genus Cecropia, which has stablished a symbiotic relationship with ants of the genus Azteca, who protects these plants – to know more: Plants and animals can also live in marriage-.
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Mechanic defence with thorns (Author: Karyn Christner, Flickr, CC).

It is known that chemical defence is ubiquitous, and thus, a lot of interactions among organisms can be explained for this reason. In this sense, some plants contains high levels of certain chemical elements, frequently metals or metallic components, which plays an important role in the defence, these plants are the heavy metal hyperaccumulating plants.

Heavy metal hyperaccumulating plants and their main characteristics

This plants belong to several families, thus hyperaccumulation is an independent acquisition occurring different times during the evolution. In all cases, hyperaccumulation allowed the ability to grow soils with high levels of heavy metals and to accumulate extraordinary amounts of heavy metals in aerial organs. It is known that the concentration of these chemical elements in hyperaccumulating plants can be 100 – 1000 times higher than in non-hypperaccumulating plants.

Generally, chemistry describes heavy metal as transition metals with atomic mass higher than 20 and with a relative density around 5.  But, from a biological point of view, heavy metals or metalloids are elements which can be toxic in a low concentration. Even though, hyperaccumulating plants has become tolerant, i.e., they hypperacumulate this heavy metals without presenting phytotoxic effects (damage in plant tissues due toxicity).

In this sense, there are three main characteristics typically present in all hyperaccumulating plants:

  • Increased absorption rate of heavy metals.
  • Roots that perform translocation more quickly.
  • Great ability to detoxify and accumulate heavy metals in sheets.

Thus, hyperaccumulating plants are prepared to assimilate, translocate and accumulate high-levels of heavy metals in vacuoles or cellular wall. In part, it is due to the overexpression of genes codifying for membrane transporters.

The threshold values that allow to differentiate a hyperaccumulating plant from a non-hyperaccumulating one are related to the specific phytotoxicity of each heavy metal. According to this criterion, hyperaccumulating plants are plants that when grown on natural soils accumulate in the aerial parts (in grams of dry weight):

  • > 10 mg·g-1 (1%) of Mn or Zn,
  • > 1 mg·g-1 (0,1%) of As, Co, Cr, Cu, Ni, Pb, Sb, Se or Ti
  • or > 0,1 mg·g-1 (0,01%) of Cd.
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Minuartia verna, copper hyperacumulating plant (Autor: Candiru, Flickr, CC).

THE ORIGIN OF HYPERACCULATING PLANTS AND THEIR IMPLICATIONS

Till the moment, several hypothesis has been proposed to explain why certain plants can hyperaccumulate heavy metals:

  • Tolerance and presence of metals in soils.
  • Resistance to drought.
  • Interference with other neighbouring plants.
  • Defence against natural enemies.

The most supported hypothesis is “Elemental defence”, which indicates that certain heavy metals could have a defensive role against natural enemies, such as herbivores and pathogens. So, in the case these organisms consume plants, they should present toxic effects, which would lead them to die or at least to reduce the intake of this plant in future. Even though heavy metals can act through their toxicity, this does not guarantee plants will not be damaged or attacked before the natural enemy is affected by them. For this reason, it is still necessary a more effective defence which allow to avoid the attack.

In contrast, according to a more modern hypothesis, the “Joint effects”, heavy metals could act along with other defensive organic components giving rise to a higher global defence. The advantages of inorganic elements, including heavy metals, are that they are not synthetized by plants, they are absorbed directly from the soil and thus a lower energetic cost is invested in defence, and also they cannot be biodegraded. Even though, some natural enemies can even avoid heavy metal effects by performing the chelation, i.e., using chelators (substances capable of binding with heavy metals to reduce their toxicity) or accumulating them in organs where their activity would be reduced. This modern hypothesis would justify the simultaneous presence of several heavy metals and defensive organic components in the same plant, with the aim to get a higher defence able to affect distinct natural enemies, which would be expected to do not be able to tolerate different element toxicity.

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Thlaspi caerulescens, zinc hyperaccumulating plant (Autor: Randi Hausken, Flickr, CC).

On the other hand, it has been shown that certain herbivores have the ability to avoid the intake of plants with high levels of heavy metals, doing what is called “taste for metals“. Although this is known to occur, the exact mechanism of this alert and avoidance process is still uncertain.

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Solanum nigrum, cadmium hyperaccumulating plant (Autor: John Tann, Flickr, CC).

Additionaly, even tough heavy metal concentration in plant are really high, some herbivores manage to surpass this defense by being tolerant, i.e., their diet allows them to intake high dosis of metals and, thus, consume the plant. This could lead to think some herbivores could become specialist in the intake of hyperaccumulating plants, and, thus, this type of defence would be reduced to organisms with varied diets, which are called generalists. It has been demonstrated to not be true, as generalists herbivores sometimes present a higher preference and tolerance for hyperaccumulating plants than specialist organisms.

For all these reasons, it can be said that evolution is still playing an important role in this wonderful weaponry war.

Difusió-anglès

 REFERENCES

  • Boyd, R., Davis, M.A., Wall, M.A. & Balkwill K. (2002). Nickel defends the South African hyperaccumulator Senecio coronatus (Asteraceae) against Helix aspersa (Mollusca: Pulmonidae). Chemoecology 12, p. 91–97.
  • Boyd, R. (2007). The defense hypothesis of elemental hyperaccumulation: status, challenges and new directions. Plant soil 293, p. 153-176.
  • Boyd, R. (2012). Elemental Defenses of Plants by Metals. Nature Education Knowledge 3 (10), p. 57.
  • Laskowski, R. & Hopkin, S.P. (1996). Effect of Zn, Cu, Pb and Cd on Fitness in Snails (Helix aspersa). Ecotoxicology and environmentak safety 34, p. 59-69.
  • Marschner, P. (2012). Mineral Nutrition of Higher Plants (3). Chennai: Academic Press.
  • Noret, N., Meerts, P., Tolrà, R., Poschenrieder, C., Barceló, J. & Escarre, J. (2005). Palatability of Thlaspi caerulescens for snails: influence of zinc and glucosinolates. New Phytologist 165, p. 763-772.
  • Prasad, A.K.V.S.K. & Saradhi P.P. (1994).Effect of zinc on free radicals and proline in Brassica and Cajanus. Phytochemistry 39, p. 45-47.
  • Rascio, N. & Navari-Izzo, F. (2011). Heavy metal hyperaccumulating plants: How and why do they do it? And what makes them so interesting?. Plant Science 180 (2),p. 169-181.
  • Shiojiri, K., Takabayashi, J., Yano, S. & Takafuji, A. (2000) Herbivore-species-specific interactions between crucifer plants and parasitic wasps (Hymenoptera: Braconidae) that are mediated by infochemicals present in areas damaged by herbivores. Applied Entomology and Zoology 35, p. 519–524.
  • Solanki, R. & Dhankhar, R. (2011). Biochemical changes and adaptive strategies of plants under heavy metal stress. Biologia 66 (2), p. 195-204.
  • Verbruggen, N., Hermans, C. & Schat, H. (2009). Molecular mechanisms of metal hyperaccumulation in plants. New Phytologist 181 (4), p. 759–776.
  • Wenzel, W.W. & Jockwer F. (1999). Accumulation of heavy metals in plants grown on mineralised soils of the Austrian Alps. Environmental pollution 104, p. 145-155.

Islands as natural laboratories for evolution

Islands are natural laboratories where we can study evolution in vivo. Whether from volcanic or continental origin, the fact that islands being isolated from the mainland by the sea makes that island biota present spectacular adaptations, sometimes originating giant or dwarf species in comparison with their mainland relatives. In this article, we describe the evolutionary mechanisms behind this phenomenon and talk about some striking examples.

Islands can have a volcanic origin, involving the emergence of virgin lands that will be colonized involving new adaptations to the new conditions. Islands can also have a continental origin, involving the separation of the mainland by tectonic processes and isolation of fauna and flora before connected.

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Volcanic conus aspect in Hawaii. Source: Steve Juverston, via Flickr.

EVOLUTION MECHANISMS ACTING IN ISLANDS

Generation of new species caused by the emergence of a geographic barrier, such as the emergence of a range, changes in sea level or emergence of new islands by tectonic movements is a process known as allopatric speciation and is the main process acting on islands. We can described two kinds of allopatric speciation:

  1. Vicariant speciation: when two populations are separated by a geographic barrier, for example when a piece of land separated from the mainland. An example is the island of Madagascar, that when separated from Africa left the biota of the island isolated from the continent by the sea.
  2. Peripatric speciation: a new population establishes and gets isolated in a new environment by a very small number of individuals from a larger population. This is the case of the colonization of a sterile land, such as oceanic islands. In this case, the individuals that colonize the new environment may not represent the genetic pool of the original population and with time and reproductive isolation; may originate a new species (founder effect).

The great British naturalist and creator of the theory of evolution, Charles Darwin, insipirated on their findings into the volcanic archipelago of the Galapagos to develop his great theory, paradigm of modern science.

Oceanic islands are formed by exploding volcanoes or movements of the mid-ocean ridge. Due to this volcanic activity, groups of islands are formed, each island having its own history, climate, topography and geology. This creates a perfect scenário to observe how evolution works because each population reaching a new island is affected by different environmental pressures and may never come in contact again with other islands populations, forming unique species, endemic to each island. Many naturalists and scientists have studied the evolution in vivo in volcanic origin archipelagos such as the Hawaiian Islands, Seychelles, Mascarene Islands, Juan Fernandez archipelago or Canary Islands. One of the last islands appeared in the Atlantic Ocean is the Suerty Island, emerged at 1963 30 km southwards of Iceland. Since then, life advent has been studied to understand ecological and evolutionary mechanisms acting in island colonization.

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Suerty Island in eruption, in the south of Island. Source: Wikimedia.

ISLANDS ADAPTATIONS: GIGANTISM AND WOODINESS

Often oceanic islands, present no predators and this triggers the appearance of very curious adaptations. One of the most surprising processes is gigantism in animals or woodiness acquisition in plants.

Woodiness acquisition in islands by herbaceous plants on the continent has been documented in several families and islands around the world. The cause of this phenomenon would be the absence of herbivores and competitors in sterile islands, which would allow developing a greater height willing to reach sunlight.

For example, in Hawaii we found the alliance of the Hawaiian silverswords. It comprises 28 species in three genus (Argyroxiphium, Dubautia and Wilkesia), all woody members of the Asteraceae family or sunflowers. Their closest relatives are perennial herbs in North America.

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Hawaiian silversword aspect from Argyroxiphium genus (left) and their closest relatives in mainland (right), from Raillardella genus. Source: Wikimedia.

In the Canary Islands, there are many examples of this phenomenon. Echium genus of Boraginaceae or borage and forget-me-not family contains about 60 species, of which 27 are located in different islands of volcanic origin in the Macaronesia (Canary Islands, Madeira and Cape Verde). Almost all members of this genus found in Macaronesia are bushes, forming an inflorescence that can reach up to three meters high, being the symbol of the Teide National Park (called tajinastes) while his nearby relatives are Eurasians herbs such as blueweed (Echium vulgare).

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Echium wildpretii (left) in Tenerife and one of its closest relative from mainland (Echium vulgare) on the right. Source: Wikimedia.

Also in the Macaronesia, we find another example in the Euphorbiaceae family. Euphorbia mellifera, endemic to the Canary Islands and Madeira and E. stygiana endemic to Azores are endangered or critically endangered trees according to the IUCN, which can grow up to 15 meters high, being part of the laurisilva vegetation, a subtropical humid forest typical from Macaronesia. Their nearest relatives are Mediterranean herbaceous species.

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Euphorbia mellifera in Maderia (left) and one of his closest relatives from the Mediterraneum basin (right, E. palustris). Source: left Laia Barres González and right Wikimedia.

In the animal kingdom, we also find peculiar adaptations. Herbivorous inhabiting islands usually have no predators or competitors, triggering appearance of larger species than in the mainland, where large carnivores avoid this characteristics incompatibles with hiding or escaping.

One of the most famous examples of island gigantism are the Galapagos giant tortoises (Chelonoidis nigra complex), including about 10 different species, many endemic to a single island of the archipelago. This turtles are the most long-lived and largest in the world. They can reach two meters in length and 450 kg in weight and can live more than 100 years.

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Galapagos giant tourtle. Source: Wikipedia.

Also among the reptiles, there are the Gallotia giant lizards of the Canary Islands. There are several single island endemic species: G. auaritae in La Palma, believed extinct until the discovery of several individuals in 2007, G. bravoana in La Gomera, G. intermedia in Tenerife, G. simonyi in El Hierro and G. stehlini in Gran Canaria, among others. Among the giant lizards of the Canary Islands there is the extinct Gallotia goliath, reaching up to 1 m length and currently being included in the G. simony circumscription.

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Gallotia stehlini in Gran Canaria. Source: El coleccionista de instantes Fotografía & Vídeo via Flickr.

Another example is Flores island in Indonesia, where we found a giant rat (Papagomys armandvillei) doubling the common rat in size. Interestingly, hominid fossils having experiences the contrary process were also found in this island, since it was dwarf primate compared to the Homo sapiens current size. It is Homo floresiensis, who was only 1 meter tall and weighed 25 kg. It became extinct about 50,000 years and coexisted with Homo sapiens.

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Giant rat (Papagomys armandvillei) from Flores. Source: Wikimedia.

Dwarfism is another evolutionary process that may occur on islands caused by the lack of resources in some islands, compared to mainland.

Unfortunately, islands holds a peculiar and unique biota that is suffering from of exploitation and extinction. The islands conservation biology helps to understand and preserve this natural heritage so rich and unique.

Laia-anglès

REFERENCES

Barahona, F.; Evans, S. E.; Mateo, J.A.; García-Márquez, M. & López-Jurado, L.F. 2000. Endemism, gigantism and extinction in island lizards: the genus Gallotia on the Canary Islands. Journal of Zoology 250: 373-388.

Böhle, U.R., Hilger, H.H. & Martin, W.F. 2001. Island colonization and evolution of the insular woody habit in Echium L. (Boraginaceae). Proceedings of the National Academy of Sciences 93: 11740-11745.

Carlquist, S.J. 1974. Island biology. New York: Columbia University Press.

 Foster, J.B. 1964. The evolution of mammals on islands. Nature 202: 234–235.

Whittaker, R.J. & Fernández-Palacios, J.M. 2007. Island biogeography: ecology, evolution, and conservation, 2nd edn. Oxford University Press, Oxford.

Shell evolution with just four fossil turtles

Turtles are charming animals yet, while they look cute to most people, they’ve been racking the brains of palaeontologists for decades. The combination of apparently primitive features and an extremely specialized anatomy, has made the reconstruction of the origin and evolution of these reptiles a nearly impossible task. In this entry we’ll try to get a general idea about the evolution of one of the most striking characteristics of turtles (the shell) with only four examples of primitive “turtles”.

CURRENT AND EXTINCT RELATIVES

As we explained in an earlier entry, the origin of turtles is still debated among the scientific community. Turtles show some anatomic characteristics not found among any current vertebrate, which makes their phylogenetic origin confusing. One of the characteristics that has puzzled palaeontologist more is their skull.

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Skull of a loggerhead sea turtle (Caretta caretta) in which we can see the lack of temporal openings. Photo by David Stang.

While the rest of reptiles are diapsid (they present a pair of temporal openings at each side of the skull), turtles present a typically anapsid cranium (without any temporal openings). Yet, recent genomic studies have proved that it’s more likely that testudines (order Testudines, current turtles) descend from a diapsid ancestor and that through their evolution they reverted back to the primitive anapsid form. What is not so clear is if turtles are more closely related to lepidosaurs (lizards, snakes and tuataras) or to archosaurs (crocodiles and birds). The most accepted hypothesis is the second one.

Even if the origins of the testudines are still somewhat mysterious, most palaeontologists coincide in that they belong to the clade Pantestudines, which groups all those species more closely related to turtles than to any other animal. A group of reptiles that are also found inside the pantestudines are the sauropterygians like plesiosaurs and placodonts.

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Reconstruction by Dmitry Bogdanov of the sauropterygian Plesiosaurus, a distant relative of turtles.

EVOLUTION OF TESTUDINES

The rest of pantestudines help us to form an image of how turtles acquired such a specialized anatomy. But first, take a look at some of the characteristics of turtles:

  • A shell made up of two parts: the upper shell (carapace) which comes from the fusion of the vertebrae and the dorsal ribs and the lower shell (plastron) that originates from ventral ribs called “gastralia” (still present in some current reptiles).
  • While the rest of vertebrates present the scapula over their ribs, the turtle’s ribs (their carapace) cover the scapula.
  • The ability to hide their heads and limbs in their shells.
  • The absence of teeth; having instead horny ridges in their jaws.

As we’ll see, these characteristics were acquired very gradually.

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The carapace of a dead turtle, in which we can see how the ribs fuse with the vertebrae to form the shell. Photo by Fritz Flohr Reynolds.

Even if their relationship with turtles isn’t still very clear, Eunotosaurus africanus is the most ancient candidate to being a turtle’s relative. Eunotosaurus was a fossorial animal that lived 260 million years ago in South Africa. This animal had very wide dorsal ribs which contacted each other, which is thought to have served as an anchoring point for powerful leg muscles, used while digging. Also, similarly to current turtles, Eunotosaurus had lost the intercostal muscles and presented a reorganization of the respiratory musculature.

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Fossil of Eunotosaurus, in which the characteristically wide ribs can be seen. Photo by Flowcomm.

The oldest indisputable relative of turtles is Pappochelys rosiane from Germany (240 million years ago). The name “Pappochelys” literally means “grandfather turtle” as, before the discovery of Eunotosaurus it was the oldest turtle relative. Just like Eunotosaurus, it presented wide dorsal ribs in contact with each other. Also, its ventral ribs were already wider and thicker and its scapular girdle was placed below the dorsal ribs.

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Drawing by Rainer Schoch of the skeleton of Pappochelys in which we can see some of its characteristics. It is believed that Pappochelys was a semiaquatic animal that swam with the aid of its long tail.

The next step in the evolution of turtles is found 220 million years ago, during the late Triassic in China. Its name is Odontochely semitestacea, which means “toothed turtle with half a shell”. This name is due to the fact that, unlike true turtles, Odontochelys still had a mouth full of teeth and it only presented the lower half of the shell, the plastron. Even if it also had thick dorsal ribs, only paleontological proofs of the plastron have been found. Odontochelys was discovered in freshwater deposits, leads us to believe that at first it only developed the plastron to protect itself from predators attacking from below.

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Reconstruction by Nobu Tamura of Odontochelys semitestacea. It’s not considered to be a true turtle due to the fact that it only had half a shell.

The first testudine known to possess a complete shell is Proganochelys quenstedti from the Triassic period, 210 million years ago. It already presented many characteristics found in current turtles: the shell was completely formed, with carapace and plastron, its skull was anapsid looking and it had no teeth. However, Proganochelys wasn’t able to retract its head and legs inside its shell (even if this may be because of the horns it had). It also presented two extra shell pieces at both sides, which probably served to protect its legs.

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Reconstruction of Proganochelys from the Museum am Lowentor of Stuttgart. Photo by Ghedoghedo.

PRESENT DAY TURTLES

The order Testudines as we know it, appeared around 190 million years ago, during the Jurassic period. These current turtles are classified into two different suborders, which both separated quickly at the beginning of the evolution of testudines:

Suborder Pleurodira: This suborder is the smallest one as it only contains three current families, all native from the southern hemisphere. The main characteristic is the form in which they retract their neck laterally inside their shell, which leaves the neck exposed and makes the cervical vertebrae present a characteristic shape (Pleurodira roughly means “side neck”). Also, pleurodirans present 13 scutes in their plastrons.

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Photo by Ian Sutton of an eastern long-necked turtle (Chelodina longicollis), a typical pleurodiran.

Suborder Cryptodira: Cryptodirans comprise most turtles. While pleurodirans only include freshwater species (as the testudines common ancestor is thought to be), criptodirans include freshwater terrapins, terrestrial tortoises and sea turtles. Apart from only presenting between 11 and 12 scutes in their plastrons, their principal characteristic is the ability to retract their neck and to hide their heads completely in their shell (Cryptodira roughly means “hidden neck”). Cryptodirans are found in practically all the continents and oceans (except in the coldest habitats).

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Photo of an Alabama red-bellied turtle (Pseudemys alabamensis) by the U.S. Fish and Wildlife Service. In this photo we can see how cryptodirans hide their heads.

Even if there still are some questions to be answered about the evolution of turtles, we hope that with this little introduction to some of the most characteristic fossil “turtles”, you have had an overall view about how turtles got their shells. Whatever their origins are, we hope that the apparition of men isn’t what puts an end to the history of this group of slow but steady creatures.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

difusio-angles

The living space of organisms

We all have our own living space, the place where we feel comfortable, like we were at home. We also have our routines, habits and that list of preferences that make us unique. Each of us, ultimately, have our own ecological niche, an extensive concept for each species that share the Earth with us. From it comes an important ecological processes such as competition or speciation, a key concepts for understanding the assembly and dynamics of natural ecosystems.

INTRODUCTION

When you are asked how you would describe close people, the first thing that comes to your mind is their way of being when you’re with them and what they loves to do. We know what is the first thing they always ask in a restaurant, what annoys them, what sites they like to frequent, what they like to do when they have free time and even how they behave when they like someone. If we have also lived with them, we could guess almost their daily routine since they wake up until they go to bed. Although we do not always have the same behaviour, there are many traits, hobbies and routines that characterize and differentiate us. Each of us have our comfort zone, our hobbies, food preferences and people with whom we love spending our free time.

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The dietary preferences of each of us and our routines and hobbies serve as a comparison to illustrate the diversity of ecological niches in the natural world. Source: Flickr, George Redgrave.

THE ECOLOGICAL NICHE OF A SPECIES

This “living space” that all of us have and in which we feel identified, is also comparable to the ecological niche of the organisms. The ecological niche of a species is a concept that always has been presented us as the “occupation”, “profession” or “work” that an organism carries up in the place where it lives (Wikipedia or CONICET), but the definition includes more than that. Hutchinson (1957) defined it as: ” n-dimensional hypervolume, where the dimensions are environmental conditions and resources, that define the requirements of a species to persist over time.” Despite the confusing definition, it is interested to point out the term “n-dimensional” as the ecological niche is based on this idea. An ecological niche is nothing more than all those multidimensional species requirements. In other words, the ecological niche of a species would be everything that involve the species and make it to prosper and survive where it is. Refers, ultimately, to all those variables that affect them in their daily lives, both biological variables -the contact with other species- and the physical and chemical ones-the climate and the habitat where they live-. An ecological niche of a species would be the spectrum of food it eats or can consume, the time of the day in which it is active to perform its functions, the time of the year and the way it carries out the reproduction, the predators and preys, the habitat it tolerates and all those physical and chemical factors that allow this species to remain viable.

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These 5 species of warblers of North America seem to occupy the same habitat (the fir), but actually not. The truth is that each warbler occupies a different position in the tree. Source: Biology forums.

To give an illustrative example, let us place ourselves in the African savannah. The main grazing ungulates and those which perform mass migrations are compound by zebras, wildebeest and Thomson’s gazelles. At first glance, you might think that their ecological niche is very similar: same habitat, same routine, same predators and same food. The same food? Absolutely not. During migration, zebras go ahead, devouring tall grass, which is the worst quality. They are followed by wildebeest, which eat what remains standing, and these are followed by Thomson gazelle, which eat the high-quality grass, which is starting to grow again.

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Although at first glance it may seem that feed on the same food, each species focuses on a different part of the plant. Source: Abierto por vacaciones.

CAN TWO SPECIES LIVE TOGETHER WITH THE SAME NICHE IN THE SAME PLACE?

The competitive exclusion principle, proposed by Gause (1934), states that two species occupying the same niche can not coexist in the long term as they come into competition for resources. Thus, in a competitive process for the same ecological niche, there is always a winner and a loser. In the end, one of the competitors is imposed by another, and then two things can happen: the extinction of the loser one (image A) or a traits displacement in order to occupy another niche (image B). In fact, the competitive exclusion principle is behind the current problems with invasive species. Invasive species niche is very similar to native species niche and, when they converge in the same habitat, the invasive species end up displacing native species, as they are better ecological competitors. It also often happens, of course, the opposite: the exotic species is worse than its counterpart and the competitor fails to thrive in the new environment.

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Image A | This study was conducted in order to observe the effect of competitive exclusion in two species of protists. Both species occupy almost identical ecological niches, but they are not living together in nature. The density of one falls sharply when they are forced to share the same space, until it eventually disappears. This same process occurs with invasive species. Source: Jocie Broth.
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Image B | When the 3 species of Darwin’s finches (in different colors) coexist on the same island, a trait displacement occurs by competitive exclusion. Individuals from the ends tend to have very similar bill depths to those of the other species, resulting in a niche overlap and subsequent competition. The final boundaries are established thanks to this process. Source: Nature.

THE FUNCTIONAL EQUIVALENCE

We have seen that to share ecological niche is synonymous of having conflict between species. However, there is a situation in which problem do not take place. The hypothesis of functional equivalence proposed by Hubbell proclaims that if the niches are identical and the species life parameters (fertility, mortality, dispersion) are also the same, none of them has a competitive advantage over the other, and the battle ends in tables. This fact seems to occur only in a very stable ecosystem in a Panama rainforest island (Barro Colorado). Different species of trees, as having almost identical parameters of life, do not compete between them and are distributed randomly, as if the individuals of different species belong to the same species. Furthermore, it seems that speciation in this kind of rainforest could also occur by chance, which would have caused the high density of species that harbor these forests.

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Tropical forests have a tree species density unique in the world. One hectare of tropical forest may contain up to 650 tree species, more than the number of tree species present in both Canada and continental US. Will Hubbell’s functional equivalence theory be behind the explanation for this curious fact? Source: Flickr, Jo.

NEW NICHES, NEW SPECIES

Speciation, or the creation of new species, usually occurs when new ecological niches are created or the existing become unoccupied. In both cases, to occupy a new ecological niche imply a gradual differentiation from the initial population to become a genetically distinct species. As an example of formation of new ecological niches we have the case of the emergence of angiosperms. Their booming opened many new possibilities, thanks both to increasing diversity of seeds and fruits (which, in turn, increased the number of specialized species) and the emergence of complex flowers, which allowed the explosion of many pollinators (facilitating the emergence of new insectivores). As an example of unoccupied niche, there is the famous case of the extinction of non-avian dinosaurs. Dinosaurs dominated a lot of niches, from land to air ecosystems, and even the aquatic environment. Those empty niches was occupied by many mammals, thanks to their high fertility and plasticity (flexibility to adapt into different habitats). That eventually led large ratios of speciation in a short time, what is known as adaptive radiation.

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This is Eomaia scansoria, an extinct species of mammals that lived at the same time as the dinosaurs. The extinction of the dinosaurs opened up a wide range of possibilities to mammals, which, although they were expanding, remained in the background. Their great plasticity led them to colonize many habitats, by occupying the free ecological niches left by the dinosaurs. Source: Wikipedia.

ASSEMBLY OF COMMUNITIES

As we have seen, the ecological niche is behind fundamental ecological and evolutionary processes. All living communities today have been formed thanks to the niches of different species. Through competition, species niches were overlaping, and the communities were assembled like a puzzle. When a piece disappears, another takes its place, playing the role that the other had in the community. However, knowing the whole ecological niche of a species is arduous and, in most cases, impossible. As in human relationships, an exhaustive knowledge of everything that influences the life of a species (or the living space of a person) is of great importance in order to ensure their long-term preservation.

REFERENCES

Ricard-anglès

Check the evolution in your own body

42% of the US population and 11.5% of the Spanish people do not believe in evolution. However, there are different evidence that Darwin was right, some of them in your own body. Have you had your appendix or wisdom teeth removed? Find out in this post which vestigial organs you have inherited from your ancestors.

WHAT ARE VESTIGIAL STRUCTURES?

Vestigial structures (often called organs althouth they are not organs properly) are body parts that have been reduced or have lost its original function during the evolution of a species. They can be found in many animals, including humans.

Esqueleto de orca en el que se observan vestigios de las extremidades traseras. Foto: Patrick Gries
Orca skeleton in which vestiges of the hind limbs can be seen. This is a proof of its terrestrial origins. Photo: Patrick Gries

Vestigial structures were fully functional in the ancestors of these species (and in the homologous structures of other existing species), but currently its function is practically useless or it has changed. For example, the second pair of flying wings in some insects such as flies have lost their function and they have been reduced to balance organs (halteres). If you want to know more about the evolution of flight in insects click here.

Besides physical structures, vestigial features can also manifest itself in behavior or biochemistry processes.

WHY ARE THEY  EVIDENCE OF EVOLUTION?

Natural selection acts on species favoring features that increase their survival and eliminating the ones with no benefits, for example when changes appear in the habitat. Individuals with unfavorable characteristics will die or will breed less and that feature will be removed after some generations, while favorable traits will remain as their carriers can pass them to the next generation.

Sometimes there are features that are neither favorable nor unfavorable, so they continue appearing in the next generations. But all has a cost structure (energy, risk to become infected, develop tumors…), so selective pressure continues acting to eliminate something that is not conducive to the success of the species. This is the case of vestigial structures, which “take longer” disappear throughout evolution. Their existence reveal that in the past these structures had an important role in our ancestors.

FIND YOUR VESTIGIAL TRAITS

THE NICTITATING MEMBRANE

We talked about it in How animals see the world. The third eyelid is a transparent or translucent membrane that protects and moisten the eye without losing visibility. It is common in amphibians, reptiles and birds. Among primates, it is only functional in lemurs and lorises.

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Nictitating membrane or third eyelid of a masked lapwing (Vanellus miles). Photo: Toby Hudson

In humans the plica semilunaris is a remnant of the nictitating membrane. Obviously we can not move it but still has some lacrimal drainage function and helps on the eye movement (Dartt, 2006).

Plica semilunaris (pliegue semilunar). Foto: desconocido
Plica semilunaris. Photo: unknown

DARWIN’S TUBERCLE AND EAR MUSCLES

10% of the population has a thickening in the ear, a vestige of the common pointy ear in primates. This structure is called Darwin’s tubercle and has no function.

Variabilidad del Tubérculo de Darwin en la punta de la oreja (0= ausente). Puede presentarse en otras zonas del pabellón auditivo: ver publicación.
Variability of Darwin’s tubercle at the top of the ear (0 = absent).  Credit.
Comparación entre la oreja de un macaco y la nuestra. Fuente
Comparison between the ear of a yellow baboon (Papio cynocephalus) and ours. Credit

Also, primates (and other mammals) have mobile ears to lead the pinna toward the sound source: surely you have noticed it in your house dog or house cat. Humans (and chimps) no longer have that great mobility, although some people may move slightly pinna. It has been proven with electrodes these muscles are excited when we perceive a sound that comes from a particular direction (2002).

Auricular muscles responsible of movement of the pinna. Credit

The occipitofrontalis muscle has lost its function to prevent the head from falling, but participates in facial expression.

PALMARIS LONGUS MUSCLE

16% of Caucasians do not have this muscle on the wrist, neither 31% of nigerian people neither 4,6% of chinese people. It can even appear in one arm and not in the other or be double.

It is believed that this muscle actively participated in the arboreal locomotion of our ancestors, but currently has no function, because it does not provide more grip strength. This muscle is longer in completely arboreal primates (like lemurs) and shorter in land primates, like gorillas (reference).

And do you have it or not? Try it: join your thumb and pinky and raise your hand slightly.

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I have two palmaris longus in the left arm and one on the right. Photo: Mireia Querol

WISDOM TEETH

35% of people do not have wisdom teeth or third molar. In the rest, its appearance is usually painful and removal is necessary.

Yo no tengo el tercer molar. Foto: Mireia Querol Rovira
I don’t have the third molar. Photo: Mireia Querol Rovira

Our hominin ancestors had them, much bigger than ours. A recent research explains that when a tooth develops, emits signals that determine the size of the neighboring teeth. Reducing the mandible dentition and the other along evolution has resulted in reduced molars (and eventually the disappearance of the third).

Comparativa entre la dentición de un chimpancé, Australopithecus afarensis y Homo sapiens. Fuente
Comparison between the dentition of a chimpanzee, Australopithecus afarensis and Homo sapiens. Look at the reduction of the last three molars between afarensis and sapiens, Credit

THE TAILBONE

If you touch your spine till the end, you will reach the coccyx or tailbone. It is three to five fused vertebrae, vestige of the tail of our primate ancestors. In fact, when we were in the womb, in the early stages of embryo development a 10-12 tail vertebrae formation is observed.

Distintos estados en el desarrollo embrionmario humano y comparación con otras especies. Créditos en la imagen
Different stages in human embryonic development (1 to 8) and comparison with other species. Credits in the image.

Subsequently it is reabsorbed, but not in all cases: it has been reported 40 newborns with a tail.

Neonato nacido con cola. Una mutación ha evitado la inhibición del crecimiento de la cola durante la gestación. Fuente
Infant born with tail. A mutation has prevented the growth inhibition of the tail during pregnancy. Credit

Although we have no tail, currently these bones serve as anchors of some pelvic muscles.

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Tailbone position. Photo: Mireia Querol Rovira

SUPERNUMERARY NIPPLES (POLYTHELIA)

It is estimated that up to 5% of the world population has more than two nipples. These “extra” nipples can be presented in different ways so sometimes are confused with freckles or moles. They are located in the mammillary line (from the axilla to the groin), exactly in the same position as other mammals with more than two breasts (observe your house dog, for example). Usually the number of breasts corresponds to the average of offspring that has a mammal, so extra nipples would be a vestige from when our ancestors had more offspring per birth. Usual is 3 nipples, but has been documented a case of up to 8 nipples in a person.

Pezón suplementario debajo del principal. Fuente
Additional nipple below the main one. Credit

FIND YOUR VESTIGIAL REFLEXES AND BEHAVIOURS

PALMAR AND FOOT SOLE GRASP REFLEX

Surely you’ve experienced that if you bring anything into the hands of a baby, automatically he grabs it with such a force that would be able to hold his own weight. This reflex disappears at 3-4 months of age and is a remnant of our arboreal past and the way to grab the hair of the mother, as with the other current primates. Watch the next video in 1934 on a study of twins (minute 0:34):

On the feet there is also a reflex of trying to grab something when the foot of a baby is touched. It disappears at 9 months of age.

By the way, have you noticed how easily children climb on any handrails or higher zones in a playground?

GOOSEBUMPS

Cold, stress or intense emotion (eg, listening to some music) causes the piloerector muscle to raise the hair giving the skin the appearance of a plucked chicken. It is an involuntary reflex in which some hormones, like adrenaline (which is released in the mentioned situations) are involved. What utility had this to our ancestors and has in modern mammals?

  • Increasing the space between the skin and the external surface, so that hot air trapped between hair helps on maintaining temperature.
  • Looking bigger to scare off potential predators or competitors.
Chimpancé con el pelo erizado durante un display antes de un conflicto. Foto: Chimpanzee Sanctuary Northwest
Chimpanzee with hair bristling in a display before a conflict. Photo: Chimpanzee Sanctuary Northwest

Obviously we have lost hair in most parts of the body, so although we retain the reflex, it has no use to us or to keep warm or to ward off predators. The hair has been preserved abundantly in areas where protection is necessary or due to sexual selection (head, eyebrows, eyelashes, beard, pubis…), but in general, can also be considered a vestigial structure.

There are more vestigial structures but in this post we have focused on the most observable. In future posts we will discuss other internal structures, like the famous appendix or vomeronasal organ.

REFERENCES