Sometime around 2 million years ago, a group of bipedal hominins in Eastern Africa gradually evolved into something that looked and acted enough like us to be part of our genus, Homo. This was an important moment in the evolutionary history of our species, but paleoanthropologists aren’t sure yet exactly which species actually gave rise to our branch of the hominin family tree. A new study, however, suggests that we can probably rule out one of the contenders.

Where did we come from?

The top contenders include a species called Australopithecus sediba, known from the fossilized remains of two adults and four children who apparently fell to their deaths in Malapa Cave around 1.9 million years ago. The other top contender is called A. afarensis, best known from the 3.2-million-year-old skeleton nicknamed 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 still built for climbing trees. So which species is actually our ancestor, and which is just a distant cousin?

“Knowing which species gave rise to Homo is important in its own right,” University of Chicago Paleoanthropologist Andrew Du told Ars Technica. “That is, it directly addresses the question of where we came from, which is of interest to all humans.” Understanding which species produced our lineage, and when and where that happened, could tell us some important things about the environments, selective pressures, and adaptive abilities that shaped us.

Alas, there’s no Pliocene version of Jerry Springer waiting in the wings with a DNA test; the fossils are so old, in fact, that recovering DNA hasn’t been possible. To try to figure out the relationships, paleoanthropologists have to look at how skeletons have changed and try to trace a path from one set of features to the next. That’s challenging enough, but the fossil record seems to preserve only around seven percent of the primate species that have ever lived, including hominins, and the record is especially spotty from around 3 to 2.5 million years ago. It’s easy to see why there’s so much debate.

One species has an alibi

If we’re going to claim a species as our ancestor, the relationships also have to make chronological sense. You’d expect a species to be older than its descendants, unless somebody finds Dr. Strange’s fingerprints in the next rock formation over. But evolution is complicated, and it’s possible for a species to split into two evolutionary branches (the fancy scientific term for this is "budding cladogenesis”). In that case, the original species could survive well after the branching point, so you could find fossils from that species that are actually younger than members of the species that branched off.

Some paleoanthropologists argue that’s what happened with A. sediba and the first members of Homo. The only known fossils of A. sediba came from a 1.97 million-year-old layer of rock at a single site at Malapa, South Africa—about 0.8 million years younger than the earliest fossil that seems to clearly fit into our genus, Homo (a 2.8 million-year-old jawbone).

While it’s technically possible for evolution to create this pattern, is it actually likely? If you put a bunch of fossils in a bag and start pulling them out at random, what are the odds that the first fossil you would draw from one species would be 800,000 years younger than the first fossils of a species that “budded” from it? That’s the question Du and co-author Zeresenay Alemseged put to the test.

Profberger

Du and Alemseged 2019

Du and Alemseged 2019

Dave Einsel / Getty Images

Brett Eloff courtesy Profberger and Wits University

They turned to the hominin fossil record and plotted the ages of the first fossils discovered from 28 pairs of species and their descendants. It’s worth mentioning that the first fossil from the ancestor species was older than the first fossil from the descendant species in just one case: Homo erectus and its proposed descendant H. antecessor. The gap there was only about 0.1 million to 0.2 million years.

In fact, Du and Alemseged calculated that, if a species lived for roughly 0.97 million years (an average calculated by a 2016 study), the odds of a fossil specimen being 0.8 million years younger than its alleged descendant were about 0.9 percent. If you’re a statistics buff, that means that if you plot the observed difference in age between fossils on a bell curve, the 0.8-million-year gap between A. sediba and the earliest Homo is hanging out on the far right tail of the curve. While it’s still technically possible for A. sediba to be the proud ancestor of our genus, it’s really, really unlikely, according to Du and Alemseged.

More fossils, please

If they’re right, that leaves A. afarensis—Lucy and her kin—as the most likely candidate that we know about so far. But we may not know all of the candidates because of those pesky gaps in the fossil record. “It is definitely possible that we haven't discovered the ancestor of Homo yet,” Du told Ars. “However, out of the currently discovered suite of species, we believe that A. afarensis is the most likely ancestor of Homo.”

To know for sure whether the A. sediba who died at Malapa are just really late survivors of their kind or whether they’re really different branches of the family tree from us, scientists need a lot more fossils to fill in those gaps and paint a clearer picture. When we’re trying to identify our species’ distant ancestors, we can only talk about probability, at least for now.

Science Advances, 2019. DOI: 10.1126/sciadv.aav9038; (About DOIs).