WEIT 2012 Chap8

WHAT ABOUT US?

What is Darwin's theory of Evolution?

The theory of Darwin suggested that all living creatures are related to one another. The fishes, the birds, the mammals, the plants, and so on all have a common ancestor. There might just have been modifications and mutations that came along the long process of evolution.

Where were the first Humans found?

Darwin supposed that the early humans first lived in Africa, then migrated out of Afica and then into Asia about 2 million years ago. And then eventually reached Europe.

What are the human's ancestors?

Darwin thought that homosapians came from Africa because our closest relatives, gorillas and chimpanzees are found there. But the author is sceptic about it, because there are no concrete evidences, no fossils to prove that. Darwin affirmed that humans had evolved from apes. He said that the mutation of the apelike species, which included the higher mental faculties of humans could be explain by the natural selection.

Who discovered the first fossil ancestors?

In 1871, the physician Eugene Dubois had discoverd in Java a human fossil. The skull was more robust than humans and the size of the brain was smaller. This fossil was too humanlike to be the missing link between us and apes. But distressed by the opposition between religion and science, Dubois decided to buried the fossil of Homo erectus in his house and he hid it for three decades.

In 1924, Dart a professor of anatomy at the University of Witwatersrand, but also an amateur of anthropology had discovered one of the greatest fossil. While he was at a wedding, a postman brought him two boxes containing bones fragment. When he oppened the boxes, he discovered a replica of a brain three time larger than a baboon's brain and bigger than a brain of an adult chimpanzee. The size of this replica of the brain was not big enough to be a primitive man, but it was much bigger than ape's brain. Dart is the one who discovered the first Australopithecus africanus.

Australopithecus africanus

The Australopithecus africanus had been found in southern Africa, especially in Taung, Sterfontein, Makapansgat and Gladysvale. This species is a fossil hominid, wich lived between 3.5 and 2.5 years ago in Pliocene.It was thought that the Australopithecus africanus was the direct ancestor of moderns humans, because of its human-like cranium which permits a larger brain. It was a bipedal hominid, with arms larger than the legs. Its fingers were more primitive, it was curved, which shows that it had the ability to climb trees and may spent some time in it.

Donald Johanson, a paleoanthropologist from America, made the decisive find by discovering "Lucy" in the Afar region of Ethopia. On November 1974, he found the bones of a single indiviual, Lucy was a female aged between twenty and thirty years old, measuring three and a half feet ang weighing around sixty pounds and the most important thing, she was a biped. Johanson discovered a new species: the Austrlopithecus afarensis

Australopithecus afarensis

The Australopithecus afarensiswas a biped hominid, which lived in Africa between 3 and 4 million years ago. It had an apelike head with a chimpsized brain, but its skull showed similarities with humans. Its tooth row was semiparabolic and the canine teeth were reduced.The arms were longer than humanlike, but shorter that apelike, and their fingers were curved, wich shows that Austrlopithecus afarensis had spent some time in the trees. This species shows perfectly the transition between apes and humans: the head and the neck were apelike; in the middle, it was a "mixture" between humans and apes; and from the wais down, this species is biped, almost like modern human. The Austrlopithecus afarensis shows a fact, which was agains the convention of the time: the human posture evolved long before human's big brain.

How different are Humans from Chimpanzees?

In 1975, Allan Wilson and Mary-Claire King at the University of California, made an experiment to try to find out what differentiates us from our closest relatives, apes. They took humans and chimpanzees protein sequence, and compared them. They then found out that humans and chimpanzees, aren't actually that different, because the protein sequence only differed by only about one percent. After some time, with some more recent researches, that difference rose to about 1.5 percent. But that still stayed relatively low, which shows a great similarity between both species. Wilson and King thought that those changes only rested on a few gene mutations. But actually, what those two researchers found in 1975 is not totally accurate. Because the reason why humans and chimps are so different, is much more complicated. We can not rely totally on that 1.5 percent difference. Changes aren't only present in genes, but also in physiology, behavior, language and in the size and the configuration of the brain. What are the genes that make us humans? We cannot answer precisely this question right now, but we know that some genes had evolved rapidly and made our difference, like the one in the immune system, gamete formation, cell death, sensory perception and nerve formation.

What are the characteristics evolutionary that distinguish hominids from apes?

The change from Australopithecus to Homo

A fossil is called Homo when it has a larger brain and it is called Australopithecus when its brain is smaller. The problem with this naming is that there is already some considerable variation in brain size within a single species. The Australopithecus rudolfensis is an example of the naming problem, scientists are arguing about whether it have to be called Homo or Australopithecus. They usually call a species, which has a brain size below 600 cubic centimetres an Australopithecus and the one which has a size above 600 cubic centimetres a Homo.

The bipedalism

The bipedalism is the ability to move the most of the time with two rear limbs/legs. In a bipedal organism, like ourselves, there are four curve in the spine that allow an adjustment of the bust. The pelvis is wide, which allows a good stability during walking. The femur is linked to the pelvis in one side, and to the knee in the other. The angle between the femur and the hips keeps the centre of gravity in one place while walking, permitting an efficient fore-and-aft bipedal stride, when the angle is toward the middle, the organism is bipedal.See Appendix 1

The cranium evolution

The cranium of the modern hominid are much bigger than the apes one, which permits a larger brain. The face had became more flat, the facial angle had increased, the forehead had became higher and the occipital bone is more rounded because of the growth of the cranium to the side and up. The apes have a massive jaw and the dental arcade forms a "U" and they usually have larges canines. The humans have a jaw that is much smaller and the dental arcade is parabolic. See appendix 2

The ability to make and use tools

This ability is shared with the chimpanzees, although they can use simple tools, they cannot use more elaborate tools, which require more flexible thumbs and an erect posture, in order to free the hands. The Homo habilis, which appeared 2.5 millions years ago, was the first tool using human. His name means "handy man" and a variety of stone tools are associated with its fossils. The Homo erectus that had a brain size and a skeleton nearly equal to modern humans used complex tools, they create stones and scrapers with complex axes and forms. This species is the one responsible for the most important event in the human cultural history, it is the species who discovered how to control fire.

How Homo sapiens appeared?

The Homo neanderthalensis lived 230 000 years ago in Europe. They had bigger brains than modern humans and were very good toolmakers and hunters and there are hints which show that Neanderthals buried their dead, it may be the first inkling of human religion. But it seems that the Neanderthal fossils disappeared around 28 000 years ago. It was said that they just evolved into modern human: the Homo sapiens. But this theory seems to be incorrect and there are two more theories which propose the explanation of this event.

The multiregional theory

This theory says that Homo erectus and Homo neanderthalensis had just evolved into Homo sapiens because of the natural selection, and it happened all over the world: in Asia, Europe and Africa.

The out of Africa theory

This theory proposes that Homo sapiens appeared in Africa and moved all over the world. They had replaced Homo erectus and Homo neanderthalensis by outcompeting their food or by killing them. Genetic and fossils had approved this theory, but the scientists are still debating on those theories.

What are the different races?

Looking at the world around us, we can quickly see that depending on which part of the globe we are, people are different. In may be in terms of skin color, shape of the eyes, hair color or type, height and so on and so forth. But of course, talking about races could be a delicate topic. In fact biologists try as hard as they can to avoid the subject. Historically speaking we would understand why. It is because racial classification has gone hand in hand with racial prejudice. Races (also called "subspecies" or "ecotypes") are simply populations of a species that are both geographically separated and differ genetically in one or more traits. That applies for every single living creature, it may be plants, animals, including humans. So we clearly have different races. But having different races explains a lot of phenomenon in our world. For instance it shows that our populations were geographically separated long enough to allow some genetic divergence to occur. It is then naturally impossible to delimitate a line where a physical divergence stops of begins. It is totally arbitrary. Looking at genes, it shows even more clearly the lack of sharp differences between races. We can only weakly differentiate the genetic variation of the physical traits such as skin color and hair type. In fact there are only about 10 to 15 percent of genetic variation between "races" and the remainder of the genetic variation, 85 to 90 percent, occurs among individual "within races". Which means that races don't show their differences mainly through the genes. In fact they carry the same genes, but simply in different frequencies.

What about now?

Nowadays there are evidences, which shows that humans had involved through the time. Let's take the example of people, who are able to digest lactose. Of course, every human are born with the capacity to digest lactose, a sugar which is in the milk, because that is the main food of an infant. But after a certain age, the ability to digest lactose disappeared, because the body stop producing lactase, an enzyme which breaks down the lactose in order to absorb it more easily. This disappearance can be caused by the natural selection. In fact our ancestors did not have any source of milk, so producing the lactase, an enzyme which was not needed was useless. But in some population, mainly the one who raise cows, the pastoralists , there are people who go on to produce this enzyme during their adulthood, this production gives them a rich source of nutrition not available to the others. The persistence of lactase depends on a simple modification in the DNA, which determines if we are tolerant or intolerant to lactose. The idea, that our ancestors had, of raising cows maybe for their meat,gave them the possibility to drink their milk and afforded them a new source of food. The advantage of the ability to digest lactose can be showed by their offspring. In fact, the one who are tolerant produced 4 to 10 percent more offspring than the one who are intolerant, this is a strong selection which had increased the number of the population. This example shows that natural selection had certainly occurred the last few thousand years and it is certainly still happening now. Slowly,the genome adapts to many sources of mortality and diseases that killed our ancestors. All the discoveries in human genetics, confirms that humans are evolved mammals transformed by the same process as every form of life: the natural selection.

---

• retour à Evolution_3BIbDF10-11_2011/12
  
• retour à Accueil