PALEONTOLOGY

Tracking our origins

Since 2000, following a few decades of calm, human palaeontology is again beginning to make headlines. For example, the dominant paradigm for explaining the separation of the human branch from that of the great apes fell apart in 2002, blown to pieces with the discovery in Chad of a 7 million-year-old hominid cranium.

Franco-Chadian palaeontological mission (MPFT) lead by Michel Brunet in the Djurab desert (Chad). © Michel Brunet
© Michel Brunet
Franco-Chadian palaeontological mission (MPFT) lead by Michel Brunet in the Djurab desert (Chad). © Michel Brunet
Franco-Chadian palaeontological mission (MPFT) lead by Michel Brunet in the Djurab desert (Chad).
© Michel Brunet
The 7 million-year-old Toumaï presents pre-human features. Discovered 2 500 km west of what had until then been considered the original cradle of our race, he supplanted Lucy (3.2 million years old, Ethiopia) as the most senior of our ancestors. © Michel Brunet
The 7 million-year-old Toumaï presents pre-human features. Discovered 2 500 km west of what had until then been considered the original cradle of our race, he supplanted Lucy (3.2 million years old, Ethiopia) as the most senior of our ancestors. © Michel Brunet

This dominant paradigm, referred to as the East Side Story, had been popularized across the world by its emblematic heroine, the young Australopithetus fossil Lucy, aged 3.2 million years. When discovered in 1974 in the region of Afar (Ethiopia) by Yves Coppens, Maurice Taïeb and Donald Johannson, Lucy was the oldest known Homininae fossil. According to the East Side Story popularised by Coppens, Lucy demonstrated that the human line started in East Africa, to the east of a tectonic fault known as the Great African Rift. East of this rift, forest gave way to savannah as the climate became dryer. In the absence of trees, our ancestors from the east began walking on their two hind legs, thus becoming bipedal and marking the start of the human adventure.

Lucy trumped by Toumaï

However, the Chad cranium, unearthed by the Franco-Chadian palaeoanthropological mission led by Michel Brunet (1) and baptised Toumaï by the president of the Republic of Chad, lay 2500 km west of the supposed cradle of humanity. Certain features (dentition, position of the occipital bone in which the spinal column is anchored, inclination of the neck) are recognised as pre-human by the large majority of the scientific community, despite Toumaï's canonical age of 7 million years, recently confirmed by radiochronology. "Do you realise what this means? Lucy, who used to be called the grandmother of the human race, was closer to us in time than Toumaï," Michel Brunet explains. This again confronts us with the mystery regarding the dates of our separation from the apes. The scattered pieces of the puzzle do not fit together. Despite a handful of recent discoveries, such pieces remain extremely rare: apart from Toumaï, only two fossil pre-humans are older than 5 million years. The first is Orrorin tugenensis, found in Kenya in 2000 (hence its nickname Millennium ancestor) and about 6 million years old, whose femur undoubtedly proves that he was bipedal and belonged to the human branch. He is joined by Ardipithecus kadabba, also more than 5 million years old, found in Ethiopia in 2001.

Who is related to whom?

Palaeontologists' tasks are not facilitated by the more than elusive relations between these various Homininae. What fossils have been found are particularly fragmentary and, in certain cases, seriously deformed. Highly complex virtual imaging techniques had to be used, for example, to reconstitute the original shape of the cranium of Toumaï - one of the best conserved fossils of all - which had been deformed and fractured by pressure and movements of the surrounding sediments. From one fossil to the next, the bones that have been conserved are generally not the same. This prevents direct comparisons. The ancestors of the Homo species therefore retain part of their mystery - a mystery to which we can hope that Africa's vast and still largely unexplored fossiliferous tracts will deliver the keys.

Out of Africa

What is certain is that these discoveries have made the human branch much older. "All this is pushing back the separation of our line to 8 or perhaps 10 million years ago," says Michel Brunet, who has already left in pursuit of Toumaï's ancestor whom he hopes to find somewhere between Libya and Chad. "This makes the African stage of human history particularly long," he comments, since it was probably around 2.5 million years ago that our ancestors, already belonging to the Homo genus, left the Black Continent.

This exit from Africa and the ensuing conquest of the world are also stages in our history that are being enriched by new knowledge. Until about a decade ago most specialists thought this episode took place around 1 million years ago, and that it was the work of Homo erectus, the most ‘intelligent' species of the Homo genus. But then an exception fossil site in the Georgian town of Dmanisi began to deliver large numbers of fossils of the Homo genus dating from 1.8 million years ago, pointing to a much earlier departure from Africa. Another surprise was the anatomy of these conquerors. "They differ in several aspects from the classical morphology [of Homo erectus]," David Lordkipanidze, director of the Georgian National Museum, recently wrote. "In particular, these specimens have a very low cerebral capacity, of around 750 cm3 for the largest, and just 600 cm3 for the smallest - in other words more or less the average of Homo habilis, a more primitive species." These Europeans also have a greater number of primitive features than Turkana man, a Homo erectus 1.6 million years old found in a very good state of conservation in Kenya. Even technologically, these "Georgians" surprised the scientific community by the very primitive character of their tools. They were apparently without bifacial technology (tools fashioned on both sides) and made do with much simpler splinters and flat stones.

Why him?

All these surprises are leading certain scientists to question the dogma that the exit from Africa was the work of erectus. It has even been proposed to baptise our Dmanisi ancestor Homo georgicus, so as to distinguish him from our other forebears. Most palaeontologists refuse, however, to take this step, preferring rather to stress the large degree of variability of these primitive men and to speak of Homo erectus, largo sensu.

The question remains why of the many known species of Australopithecus and of Homo (H. habilis, H. rudolfensis, H. erectus) which populated Africa 2 million years ago just one succeeded in leaving the continent and spreading across the world. Part of the answer probably lies in the specific nature of its skeleton: the bipedality of Australopithecus, and even that of species like habilis, was probably too primitive to allow them to cross large treeless expanses, whereas erectus was apparently capable of covering large distances. We can also consider that the level of complexity of the tools played a part. David Lordkipanidze points to another factor: on the Dmanisi site researchers found the skull of an individual with resorbed tooth cavities. This means he or she must have lived toothless for several years.

Their conclusions are particularly interesting: "It is obvious that this individual would not have survived without the help of his or her fellow humans. Very likely they gave their companion the softest parts of the animals to eat. Maybe they shared already masticated food." This ‘compassionate attitude' and ‘genuinely human behaviour', which confer a great degree of cohesion on groups, could be the key to understanding the achievements of these very first humans.

Yves Sciama

  1. Michel Brunet, of the University of Poitiers (FR), is also a professor at the Collège de France where he holds the chair of Human Palaeontology.

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Terminology

Hominidae

This African group, which became individualized round about 12 million years ago, covers the members of the human line (Homininae) as well as the large anthropoid apes (or panini), that is, gorillas, chimpanzees and bonobos.


Homininae

These are all the members of the human line after the separation from the panini. As well as Orrorin, Toumaï and the two Ardipithecus (A. ramidus and A. kabbada), it also includes the Australopithecus and the members of the Homo genus. Bipedality is probably a common characteristic of this group, albeit in apparently fairly diverse forms.


Homo

The various species of the Homo genus present major morphological differences between the most primitive among them (H. habilis) and the most recent, that is Neanderthal man and modern man (H. sapiens). The latter, our own species, appeared around 200 000 years ago and is now the sole representative of the Homo genus.



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