OHS2020 Chap1

What is the definition of an hominin?

An hominin is a species belonging to the Hominina subtribe of the Hominini tribe of the Homininae subfamily. The Homininae subfamily contains the Gorillini and Hominini tribes. The Hominina subtribe contains all species more closely related to the human species that to chimpanzees. (p.7)

The expected characteristics of an hominin are smaller canins than those of chimpanzees (p.11), less curved finger bones, stronger and longer thumbs (p.14), but not an especially larger brain cavity as this derived feature is more recent than the others(p.11)

What is sexual dimorphism?

Sexual dimorphism is the presence of morphological differences (that are not just in the sexual organs) between males and females of a same species. These differences can be the size, the shape of the teeth, bones, muscles (p.15). The higher the sexual dimorphism is, the bigger the differences between the two opposite sexes are. Competition between males and polygyny are also associated with high sexual dimorphism in apes, for instance in gorillas.(p.61) --NorahF (discussion) 2 avril 2020 à 00:33 (CEST)

Definition of a derived character

A derived characters are "anatomical features that have changed from the ancestral or primitive condition."(p.15)

How do we know that our human story began 7 million years ago?

First the time when the Hominina subtribe split from the other Homininae can be estimated around 5-9 million years ago through a method called molecular clock.

With this method, we compare the human DNA to the chimpanzees' DNA and deduce the time of their divergence from the amount of mutations in some parts of the DNA where the rate of mutation is known. (p.8)

To be more precise, we use the fossil record. To date fossils, there are two types of methods: relative and radiometric dating.

Relative dating consists in situating a fossil in time using the stratum in which it has been found and/or the other fossils found with it. (p.32) For instance, biochronology, in which the other animal fossils found around the fossil we want to date are compared to well-dated fossils of the same animals, is a relative dating method. (p.22)

Radiometric dating  is based on physical properties such as radiactivity. For instance, carbon-14 is a radiometric dating method using the proportion of the carbon-14 isotope that is produced in the atmosphere and absorbed by organisms, and decays by radioactivity into carbon-12, so that its proportion starts diminishing after the organism dies and stops getting new carbon atoms from what he eats. (p.32)

Why is fossilization considered as a rare process?

It is rare because we need uncommon conditions for fossilisation to happen. The carcasses of animals are most of the time taken away by predators and scattered around, so that they become eventually unrecognisable and desintegrate because of the environment. (p.14) --ThomasG (discussion) 28 mars 2020 à 20:01 (CET)

But fossilization can occur in two ways

1. When the dead animal is covered in sediment, the bones of the dead animal will take up chemicals that will allow it to be conserved

2. When it is unaccessible for predators, for instance by falling in a cave, because then, animals cannot scatter the carcasses around. So , it can also be conserved. Note that complete skeletons can be found thanks to this kind of conservation.=== --ThomasG (discussion) 28 mars 2020 à 20:01 (CET)

Good Thomas but use the titles exclusively for questions. If you want to organize your answer with bullets, there are other ways to do it. I can show you if you want via a Meet!--Pierre.brawand (discussion) 29 mars 2020 à 14:21 (CEST)

What is the molecular clock concept?

The molecular clock is way to deduce how long ago the speciation process between two species occured from how much mutations occured in a part of the genome whose we know the rate of mutation. The biomolecular data used for these computations are usually nucleotides sequences for DNA or amino acids sequences for protein.--NaywaT (discussion) 31 mars 2020 à 18:35 (CEST)

What is speciation?

The speciation mechanism is the process by which a new species is made. The definition on a species is "a group of interbreeding natural populations that are reproductively isolated from other such groups."--NoraM (discussion) 30 mars 2020 à 10:54 (CEST) There exists many ways in which scientists determine if organisms are from the same species. One idea is that organisms from the same species resemble each other more than organisms from another species. But the problem with this kind of vision is that scientists don't always agree on the terms of that. For speciation, scientists focus on fossil records. But the main problem for them is to know how much anatomical variations can be accepted within a same species. --NoraM (discussion) 30 mars 2020 à 10:54 (CEST) One of the other main idea to create a new species is that they can reproduct and generate fertile offspring.--NoraM (discussion) 5 avril 2020 à 11:55 (CEST)

very nice Nora. Maybe the idea of generating a fertile offspring might be added in your answer. It's however nice to define right away that the definition is challenged. For example, H. sapiens and H. neanderthalis did once hybridize... and the offspring was fertile... Should we then considered these two species as one single species...?Pierre.brawand (discussion) 30 mars 2020 à 12:11 (CEST)

Is sexual dimorphism relevant to the recognition of hominin species ?

Sexual dismorphism is a difference in shape and size between males and females organisms in a single species. Sexual dismorphism can induce a variation in a species and those organisms can be more recognisable than in species where males and females are more similar.--NoraM (discussion) 30 mars 2020 à 11:00 (CEST)

Why is it important to compare the upper jaws of the chimpanzee, the Australopithecus and the modern human?

It is important to compare the three upper jaws in order to understand the links that may exist between the three hominin species.--Pierre.brawand (discussion) 27 mars 2020 à 18:52 (CET)

How do we reconstruct hominin diets?

To know the type of food hominins ate, scientists look at the chemical composition of body tissues such as bones. --IrisH (discussion) 24 mars 2020 à 11:20 (CET) To be more precise, scientists look for the presence of C3 or C4, which is the carbon isotope signature. If there C3, it would mean that the diet is composed of fruits, leaves , part of trees, bushes or shrubs. And if there's C4, it means that the species eat more plants that grow in savannah habitats.--ThomasG (discussion) 27 mars 2020 à 11:42 (CET) Note that the diet also shows a species' habitat because certain type of plants can only grow in certain places--ThomasG (discussion) 27 mars 2020 à 11:42 (CET)

How does the analysis of carbon isotopes work?

An isotope of carbon, carbon-14, is constantly produced in the Earth's atmosphere and is unstable. Its half-life is 5,730 years, which means the amount of it decreases by half every 5730 years. When an ecosystem relies on plants, which absorb the carbon in the atmosphere through photosynthesis, all the living animals it contains also absorb carbon 14 in the same proportion. When an animal dies, it stops absorbing new food and thus new carbon-14 atoms. But the carbon-14 it had absorbed keeps decaying, so that its proportion decreases through time. Knowing the rate of decay and the proportion of carbon-14 in a living animal and in the fossil, we can estimate when it stopped absorbing carbon-14, and therefore when it died. However as the proportion of carbon-14 is already very small in a living organism, this method becomes quickly less reliable for fossils older than 30'000 years and cannot be used past 50'000 years. (p.32-33) --JeremyS (discussion) 7 avril 2020 à 10:43 (CEST)

What are the advantages to be bipedal ?

Being bipedal allows you to have your hands free and therefore to reach food more easily (ex: fruits on a tree), it allows you to have a wider field of vision and to see predators more easily. It also allows you to defend yourself with your hands and to create tools. --ChloeS (discussion) 2 avril 2020 à 15:13 (CEST)

What is the difference between the mouth of humans and apes?

The apes have anatomical features linked to honing called the canine premolar honing complex, which have completely disappeared in humans. With a canine premolar honing complex, canines are sharpened on their sides, whereas in humans they wear mostly on their tips. We have different tooth proportions than apes, and a differently shaped dental arcade : smaller canines and incisors and the teeth are arranged in a more rounded dental arcade.--MicheleM (discussion) 6 avril 2020 à 17:06 (CEST)

How apes use their hands ?

Apes use their hands for a variety of tasks including grooming or food acquisition. They have long curved fingers with strong muscles that are useful for locomotion (climbing, swinging, hangig from branches, etc.). When travelling on the ground, they use their hands as well as their feet. There are two kinds of "ground walking" : the knuckle walking : walk on the backs of the fingers, used by chimpanzees and gorillas, and the fist walking : walk on the sides with hands fold into fists, for orang-utans.--MicheleM (discussion) 6 avril 2020 à 17:20 (CEST)

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