Welcome to the family - making order in the evolutionary tree of the human family / Bernard Wood

Recent molecular analyzes and fossil findings suggest that the story of human evolution is much more complex, and much more interesting, than we imagined

"So what do you think?" Berger asked me. He had just removed the lids of two large wooden boxes that each contained the fossilized bones of a humanoid skeleton from Malpa, South Africa, which had been carefully packed. These two creatures, who breathed their last two million years ago, caused quite a commotion. Most of the fossils are "isolated" finds, a jawbone of a mouth and a leg there and the scientists have to guess if the separate bones belong to the same individual. Imagine walking along a highway and finding car parts, a front fender here and a transmission there. Do they belong to the same model, or even the same manufacturing company? And maybe they didn't fall out of a car at all but out of a truck?

In contrast, the skeletons from Malpa, although imperfect, are complete enough to reduce the probability of random mixing. Like "Lucy", found in excavations in Ethiopia in 1974, and "The Child from Turkana", found in Kenya in 1984, they tell much more than partial fossils. But these skeletons made headlines not because they were perfect and so well preserved, but because Berger, a paleoanthropologist at the University of the Witwatersrand in Johannesburg, estimated that these individuals were part of a population of a creature that was a direct ancestor of our biological type (genus), Gay.

We all have ancestors. I still have an elderly parent. I had the good fortune to know all four of my grandparents, and I can even dimly remember three great grandparents. But I also have relatives who are not my ancestors. Not much, because both my father and I were only sons. I had several uncles and aunts. They are a necessary part of their descendants' genealogy, but in terms of my family's genealogy they are considered "possible additions" to the car. Berger therefore wanted me to stop marveling at the details of the teeth and jaws and tell him whether, in my opinion, the skeletons from Malpa are equivalent, from an evolutionary point of view, to my parents and my children, or whether they are equivalent to my uncles and aunts. In other words, if they belong to a population that was a direct ancestor of modern man or only his relatives.

When I first started researching East African humans, nearly fifty years ago, everyone believed that almost all of our extinct relatives were direct ancestors, and the farther we went into the past, the less human-like and more ape-like each of them became. But today we know, from genetic studies, from Neanderthal fossils and from fossils called the "Hobbits" of Flores in Indonesia (Homo floresiensis), that for the last several hundreds of thousands of years our direct ancestor shared the world with some of our relatives. Moreover, discoveries of other fossils make it clear that even in much earlier stages of our prehistory, one million to four million years ago, there were periods when other relatives walked the earth together with our ancestor. The fact that multiple branches of our family tree have lived at any given time makes identifying the direct ancestor of modern man more difficult than paleoanthropologists thought even twenty years ago. However, the challenge means that human evolution is more complex and fascinating than most of us know.

One branch or several branches?

When I entered the field in 1968, the dominant approach to the "tree of life" was Darwin's approach. He claimed that the animal world is connected in the way that the branches of a tree are connected. In Darwin's tree of life, all species alive today are found at the outer top of the tree, while all extinct species are located closer to the trunk. Every human being alive today has ancestors, and so does every species alive today. So, theoretically, the branches, or lineages, are the only ones That must To be in the tree of life are those that lead from the living species to the depths of the tree, and the only species that have become extinct That must To be in the tree are those found on these connecting branches. All others are evolutionary dead ends.

This means that as far as modern man and the apes living today are concerned, the only branches and species that should be in the part of the tree that belongs to us are the branches that connect us to the ancestor that we share with chimpanzees and bonobos. Based on molecular evidence, this ancestor lived eight to five million years ago.

In 1960, the outermost branch of the tree of life, leading to modern man, seemed fairly straightforward. At its base was the Australopithecus, an ape whose remains have been unearthed by paleoanthropologists in South Africa since the 20s. It was suggested that Australopithecus was replaced by Homo erectus from Asia, which was taller and had a bigger brain. Homo erectus also spread to Europe, evolved into Neanderthal man, who in turn continued the evolution into Homo sapiens (known as modern man). All of these were considered ancestors of modern man, i.e. equivalent to my parents, my grandparents and their parents. Only one type among the hominins (all the creatures closer to the modern man than the chimpanzees and the bonobos, all of which, except for the modern man himself, are extinct), a species called at the time "the robust Australopithecus", due to its large jaws and chewing teeth, is considered a short-lived member of the human branch, Thus he is equal to my uncle or aunt.

This opinion changed when the focus of early hominin research shifted from southern Africa to the east following the discoveries of hominins in the Oldubai Gorge in Tanzania by Lewis and Mary Leakey. The focus was shifted not only because the trickle of discoveries in East Africa in the early 60s was a torrential stream, but also because the context of the fossil finds in East Africa, especially with regard to dating, was very different from that in the South.

The hominin fossils in the south were found in the past, and still today, mainly in caves in dolomite rocks (carbonate rock rich in magnesium). Although researchers sometimes find a well-preserved skeleton of an individual (such as those at Malpa), most of the early hominin fossils found in these caves were the remains of meals by tigers and other prey animals. Uneaten teeth and bones were washed from the surface into the cave along with dirt. From the moment they were inside the cave, the bones and dirt turned into what is called a cone of pouring, which is the sloppy version of the sand cones arranged at the bottom of traditional hourglasses. The layers of the pour cones inside the caves do not always obey the general law according to which older layers are at the bottom and younger ones at the top of the pile. And if this frustration is not enough, until recently the researchers did not know how to date the sediments in these caves. In the early 60s, all researchers could do was fit the hominin remains into a rough timeline based on the types of fossilized animals found in the caves.

The fossil remains of the hominins in East Africa have been found at sites close to the East Syrian-African fault that cuts through this part of Africa, from the Red Sea in the north to the shores of Lake Malawi and beyond in the south. The hominin fossils in East Africa were not found in caves, but in sedimentary rocks that were deposited around lakes or along river channels. Many of these rock layers preserve in their contents the direction of the Earth's magnetic field as it was when they were formed. These layers are found in sites open to the air, therefore they contain volcanic ash, emitted from the many volcanoes that formed in and around the fault zone due to the movement of the tectonic plates. All these characteristics allow researchers at each and every site to determine the age of the layer, regardless of the fossils it contains. Moreover, the layers of volcanic ash function like a series of blankets of time that cover the entire area. They allow researchers to link fossils found thousands of kilometers apart.

Many of the sites richest in hominin fossils, such as those found in the Omo-Turkana Basin and north of it, along the Awash River, have layers representing millions of years. In this way it is possible to determine the dates of the "starting point" and the "ending point" of a certain group of hominin fossils. This ability makes it clear that four million to one million years ago there were many times when more than one type of hominin lived in East Africa, not least if you compare East Africa with the south. For example, for about one million years (approximately 2.3 million years to 1.4 million years ago) two very different types of hominins - Paranthropus boisii and Homo habilis - lived in the same area of ​​East Africa. They were so different that a prehistoric safari guide would tell you that their skulls and teeth are almost impossible to confuse, no matter how fragmentary the fossil remains. It is also quite clear that the hominins of East Africa are very different from those found in South Africa, but we will deal with that later.

The presence of Paranthropus boisii and Homo habilis remains in one layer does not mean that these two species competed for the same water hole, but that at least one of them was not an ancestor of modern man. Although there is much later evolutionary evidence that indicates a small degree of mating between Neanderthals and modern humans, but in my opinion the physical differences between Paranthropus boisii and Homo habilis are much greater and indicate that mating between these two types is less likely. Even if they did occur, they failed to blur the differences between the two sexes. In other words, the image of a single branch just doesn't seem appropriate to represent humans two million years ago. Our early parentage looks more like a bundle of twigs, and maybe you can even think of it like a tangled bush.

There is also evidence of multiple dynasties in our more recent past. For example, Neanderthals have been recognized as a separate species for about 150 years, and over time researchers are discovering more and more ways in which they differ from modern humans. We know that there is another hominin, called Homo erectus, that survived longer than was initially thought, and that Homo floresiensis, although it is possible that he lived only on the island of Flores, was probably the fourth hominin to live on Earth in the last hundred thousand years. Evidence for the existence of a fifth distinct hominin, Denisovans, comes from DNA extracted from a four-hundred-thousand-year-old finger bone. And there is evidence for at least one more "ghost lineage" from a hundred thousand years ago found in the DNA of modern humans living today. And so the recent evolutionary history is much more "discourse-like" than was thought even a decade ago.

Perhaps we should not be surprised by the similarity of our evolution to a thicket of bushes. The existence of many related species at the same time seems to be a general rule in the past of many groups of mammals, so why should hominins be any different? Despite this, critics of the tangled family tree have accused paleoanthropologists of being overzealous in identifying new species from their findings, perhaps out of their desire for fame and additional research funds.

I believe we are discussing a real phenomenon. First, there are logical and solid reasons to think that the fossil record always results in an underestimation of the number of species. Secondly, we know from the animals of our time that there are species that there is no doubt about being separate and yet it is difficult to distinguish between them based on bones and teeth, the so-called hard tissues, which are the only ones that survive and turn into fossils. Furthermore, most mammal species that lived three million to one million years ago have no direct living descendants. Therefore, the existence of some early hominin species that have no direct descendants is not "strange" at all.

If there was indeed a rich diversity of hominins in the past, biologists must uncover the evolutionary pressures that caused this. The climate is one of the obvious candidates. Climate, and therefore also habitats, undergo changes over time according to different trends, and they undergo cyclical fluctuations within these trends. In general, in the period we are discussing there is a cooling and drying trend. But within this trend the climate fluctuates at predictable intervals: sometimes it is hotter and more humid, and sometimes cooler and drier. The posture, movement and diet that were effective in one period may be less successful in another period. Another pressure that may have caused the increase in hominin diversity is competition between different hominin species. If two hominins share the same habitat, even in the broadest sense, they will tend to force each other to adopt different survival strategies. This phenomenon, called trait shift, can explain how Homo habilis and P. boisii developed such different teeth and jaws, since one group preferred hard and fibrous foods, such as grass, and the other group tended to a diet that included softer, but also rarer fruits, and a meal of meat or marrow Bone from time to time. Moreover, when the hominins developed different cultures, the difference in their worldview and customs could have prevented the merging of the species following pairings.

In addition to the anatomical differences, researchers can now decipher fossils at the molecular level. However, when it comes to the early hominins, for which we do not yet have genetic evidence, it is still difficult to distinguish between hominins who look like my parents, my grandfather and their parents, and those who look like my uncles and aunts. Even if two fossils have jaws and teeth that are similar in shape, it does not mean that they share a close evolutionary history. The similarity could be due to the fact that similar environmental challenges lead to similar morphological solutions. For example, an ax can cut down eucalyptus trees in Australia as it can fell firs in northern Europe. Australians and Europeans could arrive at the same design without one group presenting it to the other. We also know that there is a limit to the ability of morphology to undergo changes. Each type of animal or plant has a finite number of anatomical or physiological solutions to a particular environmental challenge. If so, a similar characteristic in fossils from two species does not mean that they are necessarily close taxonomic members. They may simply be two relatives who developed the same physical solution to similar environmental challenges.

What does the future hold for identifying our direct ancestor? I am willing to go a step further beyond supporting the view that many hominin species roamed our planet at one time. I believe that the hominin diversity identified in the last four million years existed even before that, partly because researchers have not yet invested much effort in discovering hominins that lived in earlier times. For this reason, the number of sites that have been studied older than four million years is smaller than the number of later sites that have been studied. Admittedly, the work is hard. Hominins are the rarest of all mammal fossils found. You have to rummage through piles of pig and antelope fossils and sort them before you expect to find an accidental hominin. But if we make a concerted and deliberate effort to find them, they will surely appear.

Another reason to believe that more hominin species will be discovered is that when you look at the fossils of the more common mammals, you notice that the number of lineages from three million years ago is almost equal to those after that time. Why was it not observed that the hominins would show a similar pattern? On top of that, ancient hominin sites are no more than three percent of the land area of ​​Africa, and maybe even less than that. It is unlikely that such a small geographic sample was able to locate the evidence for all the ancient hominin species that ever lived on this continent.

However, any new find older than four million years will likely cause even greater uncertainty. The closer you get to the split between man and the chimpanzee and bonobo lineages, the harder it is to distinguish between a direct ancestor and a relative. It will also be more difficult to know for sure if it is a new hominin species, or perhaps an ancestor of chimpanzees or bonobos or even a species that belongs to a lineage that has no living representative. If paleoanthropology is difficult and challenging now - and I am not yet convinced that the skeletons from Malpa are direct ancestors of man - it will not be easier in the future. But these are the challenges that make this field so fascinating.

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in brief

Tracing the evolutionary ancestors of Homo sapiens It was once considered a simple matter: Australopithecus gave birth to Homo erectus, which gave birth to the Neanderthals, which gave birth to us.

In the last forty years, fossils from East Africa and other findings have completely shattered this hypothesis.

The most recent evidence shows that at certain times several hominin species lived together on Earth. Understanding the connections between them, and which of them led directly to us, will continue to occupy paleoanthropologists for decades to come.

About the author

Bernard Wood (Wood) is a paleo-anthropologist with a medical background at George Washington University. His interest in human evolution arose in 1968, when he was a medical student and joined Richard Leakey's expedition in northern Kenya.

More on the subject

Fossils Raise Questions about Human Ancestry. Ewen Callaway in Nature. Published online September 8, 2011.

Human Evolution: Fifty Years After homo habilis. Bernard Wood in Nature, Vol. 508, pages 31–33; April 3, 2014.

What Does It Mean to Be Human? Smithsonian Institution's Human Origins

The article was published with the permission of Scientific American Israel