The first hominins

The story

A family with many species

When scientists talk about “humans”, they usually mean a whole group of species, not just one. The group is called the hominins (or hominids, in older language), and in common usage it includes our own species and all our extinct relatives that appeared after the split from the chimpanzee line. Chimpanzees are our closest living cousins: their DNA is almost the same as ours. According to genetic calculations, the two paths separated between about 9 and 6 million years ago. The exact moment is hard to pin down, because very few fossils survive from that time.

That is why every new find from that period makes the news. The bones are rare, often crushed or broken, and found in places where the landscape has completely changed since. For the human line, before four million years ago, there are very few remains, and almost all of them are fragments.

The skull in the desert

On 19 July 2001, in the Djurab Desert in Chad, a student on the team of the French palaeontologist Michel Brunet saw something dark sticking out of the sand. It was an almost complete skull, squashed by the weight of the ground but still recognisable. The place is called Toros-Menalla, and today it is sand and wind; seven million years ago it was a landscape of lakes, grasslands and little woods along the shores. The skull was named Sahelanthropus tchadensis, “Sahel man of Chad”, and it also got a nickname in a local language: Toumaï, which means “hope of life”.

Toumaï has a mixture of features. The brain was small, about the same as a chimpanzee's, roughly a third of ours. The face, however, was short and the canine teeth were small, features that look more like hominins than like apes. The most debated part is the hole at the base of the skull where the spinal cord comes out: to many scholars, its position suggests that the head sat on top of the spine, as in someone who stands upright. Others are not convinced, and the debate is still open. A thigh bone found in the same place has also been studied recently, and a partly upright walk has been worked out from it; but some experts think the data are not enough.

Other candidates

Sahelanthropus is not the only very ancient hominin. In 2000, in the Tugen Hills in Kenya, Brigitte Senut and Martin Pickford described Orrorin tugenensis, about 6 million years old, known above all for a thigh bone whose shape suggests that it walked on two legs. In Ethiopia, in the Awash valley, Ardipithecus was found a little later: first a species of 5.8–5.2 million years ago, then Ardipithecus ramidus, of 4.4 million years ago, of which a very incomplete but precious skeleton was found, nicknamed Ardi. Its feet could still grasp branches, and its pelvis was suited to both walking and climbing.

Scholars do not all agree about which of these were really our ancestors and which were branches with no descendants. The most realistic picture is a bush, not a ladder: many species living at the same time, some of which died out without leaving a trace in our family tree.

Why stand up?

Why walk on two legs? There are many ideas and none is certain. Perhaps it helped to carry food or babies over a distance, or to see over tall grass, or to use less energy on the long walks between one tree and the next when forests were getting thinner. Probably it was several things together. What we do know is that these first hominins did not leave the trees: they walked upright for a stretch and then climbed back up. Walking upright freed the hands from a job. For millions of years the hands did not do much that was new, but they became free to carry, throw and chip stone: it is the starting point of everything we will tell in the next cards.

With Sahelanthropus this story begins in our books. Whether it is the true beginning, or a close cousin, future fossils will be able to tell.