We probably do not come from Australopithecus sediba

About 2 million years ago, a group of bipedal homininins from East Africa gradually evolved into something that looked and acted quite like us to be part of our genre. Homo. It was an important moment in the evolutionary history of our species, but paleoanthropologists do not yet know exactly which species actually gave birth to our branch of the family tree of the hominin family. A new study suggests, however, that we can probably exclude one of the candidates.

Where did we come from?

Major competitors include a species called Australopithecus sediba, is known by the fossilized remains of two adults and four apparently dead children in the Malapa cave about 1.9 million years ago. The other most important competitor calls themselves A. afarensis, best known from the 3.2 million year old skeleton dubbed Lucy and a set of preserved footprints near Laetoli, Tanzania.

Both species walked on two legs and probably made stone tools, but their shoulders, arms and hands were also built to climb trees. So what species is really our ancestor and which is only a distant cousin?

"To know which species gave rise to Homo is important in itself, "University of Chicago paleoanthropologist Andrew Du said at Ars Technica. "That is, it directly addresses the issue of origin, which interests all humans." Understanding what species produced our lineage, and where and when that happened, could tell us more about the environments, the selective pressures and the adaptability that shaped us.

Unfortunately, Jerry Springer does not have a Pliocene version waiting for a DNA test; the fossils are so old, in fact, that recovering the DNA was not possible. In an attempt to understand relationships, paleoanthropologists must consider how skeletons have changed and try to trace the path from one set of entities to another. It's quite difficult, but the fossil record only seems to hold about 7% of the primate species that have ever existed, including hominins, and the inventory is particularly erratic about 3 to 2.5 million years ago. years. It's easy to see why there is so much debate.

A species has an alibi

If we want to claim a species like our ancestor, relationships must also take on a chronological sense. You expect one species to be older than its descendants, unless someone finds Dr. Strange's fingerprints in the next completed rock formation. But the evolution is complicated and it is possible that one species divides into two evolutionary branches (the scientific term of fancy designating this term is "nascent cladogenesis"). In this case, the species of origin could survive well after the point of branching. fossils of this species that are actually younger than the members of the species that has branched out.

Some paleoanthropologists maintain that this is what happened with A. sediba and the first members of Homo. The only known fossils of A. sediba come from a 1.97 million year old layer of rock located on a single site in Malapa, South Africa, about 0.8 million years old younger that the oldest fossil that clearly seems to belong to our genre, Homo (a jaw 2.8 million years old).

Although it is technically possible for evolution to create this model, is it really likely? If you put a pile of fossils in a bag and start extracting them at random, what are the chances that the first fossil that you would extract from a species is 800,000 years younger than the first fossils of a species? species that has "sprouted"? ? This is the question Du and his co-author, Zeresenay Alemseged, put to the test.

They turned to the hominins archives and traced the ages of the first fossils discovered in 28 pairs of species and their descendants. It should be mentioned that the first fossil of the ancestor species was older than the first fossil of the descending species in only one case: Homo erectus and his proposed descendant H. antecessor. The gap there was only about 0.1 million to 0.2 million years ago.

In fact, Du and Alemseged calculated that, if a species lived for about 0.97 million years (an average calculated by a 2016 study), the odds that a fossil specimen would be 0.8 years younger than his presumed descendant were about 0.9%. If you are passionate about statistics, it means that if you trace the observed age difference between fossils on a bell curve, the gap of 0.8 million years between A. sediba and the sooner Homo is hanging at the far right of the curve. Although it is still technically possible to A. sediba to be the proud ancestor of our kind, it's really, really improbable, according to Du and Alemseged.

More fossils, please

If they are right, that leaves A. afarensis-Lucy and his loved ones – as the most likely candidate we knew until now. But we might not know all the candidates because of these saturating gaps in the fossil record. "It is quite possible that we did not discover the ancestor of Homo yet, says Du to Ars. "However, among the series of species currently discovered, we believe that A. afarensis is the most likely ancestor of Homo. "

To know for sure if the A. sediba who died in Malapa are truly end-of-life survivors or truly different from our family tree, scientists need many more fossils to fill these gaps and paint a clearer picture. When we try to identify distant ancestors of our species, we can only speak of probability, at least for the moment.

Progress of science, 2019. DOI: 10.1126 / sciadv.aav9038; (About DOIs).