« OHS2020 Chap1 » : différence entre les versions
(j'ai rajouté des infos sur les méthodes de datation) |
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=How do we know that our human story began 7 million years ago?= | =How do we know that our human story began 7 million years ago?= | ||
First | 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?= | =Why is fossilization considered as a rare process?= | ||
Version du 30 mars 2020 à 12:54
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)
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 several conditions to fossilisation to happen. Carcasses of animals can be taken away by predators and some part of the carcass is being scattered around and they become eventually unrecognisable because of the environnement that desintegrates it.--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, such as caves, because then, animals cannot scatter the carcasses around. So , it can be conserved also. Note that complete skeletons can be found thanks to this 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 analyse the time that occured between the speciation process among to species.
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)
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)
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