The oldest hominin fossil ever found in the Levant

When the first members of our species ventured out of Africa, they walked into a world that earlier hominins, such as Homo erectus, had first explored a million years earlier. According to a recent study of a 1.5 million-year-old vertebra, those earlier hominins may have expanded beyond Africa in several waves—each following different environments and equipped for different ways of life.

Taking a second look

Anthropologists found a single vertebra from the lower back of a hominin child who died 1.5 million years ago. Mixed with the fossilized bones of hippos, mammoths, giraffes, saber-toothed tigers, and warthogs, the bone had sat among the remains of Pleistocene fauna since the 1966 excavation that unearthed it. But when University of Tulsa anthropologist Miriam Belmaker, a co-author on the recent study, looked through the animal fossils as part of another recent study (an effort to narrow down the age of the site), she recognized the vertebra as belonging to a member of our genus, Homo.

And the fact that the pieces of the vertebra hadn’t all fused together into a single hard, bony piece meant that it came from a child who hadn’t yet finished growing and maturing. They were probably somewhere between 6 and 11 years old when they died.

This child had lived during an important moment for human evolution. Between 1.9 and 1.1 million years ago, some of the earliest members of our genus began to expand into Europe and Asia for the first time. Our species repeated a similar journey out of Africa about a million years later, but much earlier hominins did it first.

The owner of the 'Ubeidya vertebra almost certainly didn’t think of her life in those terms. The venture out of Africa wasn’t a deliberate march into the unknown, just a gradual expansion into a little bit of new territory each season. In what’s now the Jordan Valley, these early hominins lived in a warm, humid woodland. They shared the landscape with classic Pleistocene species like mammoths, saber-toothed tigers, and giant buffalo, along with animals like baboons, hippos, jaguars, and warthogs. To prepare their food, they used a style of stone tools that archaeologists call Acheulean.

Other styles

Farther north, at Dmanisi Cave in what’s now Georgia, other hominins lived in a drier, open grassland and used a type of stone tool technology now known as Olduwan. The fact that groups of hominins in different parts of Eurasia used different collections of tools, according to some paleoanthropologists, suggests that hominins left Africa in several separate waves. Each wave brought different cultural adaptations—including stone tools—with them.

However, Bar-Ilan University anthropologist Alon Barash and his colleagues say the vertebra suggests those waves of migrants may have been not just different cultural groups, but members of more than one hominin species, each even more different from the other than we were from the Neanderthals or Denisovans. That’s because, based on what the 'Ubeidya vertebra suggests about the size and growth rate of its former owner, the child seems to have developed at a different pace than the hominins at Dmanisi.

If you want to draw conclusions about a long-dead person’s stature, a single vertebra isn’t much to go on. Most of the time, anthropologists use the long bones of the arm and leg for height estimates. But when the only bone you have is a lumbar vertebra, you consult the applicable set of tables and formulas, and you make do with an estimate.

Barash and his colleagues estimated that the child at 'Ubeidya probably stood about 155 centimeters tall. That’s the average height for a 13-year-old boy or a 12.5-year-old girl in the modern US. If the child was between 6 and 11 years old when they died, as the unfused parts of the vertebra suggest, then they were pretty tall for their age. As an adult, this person probably would have stood somewhere around 198 centimeters tall—about 20 centimeters taller than the average American today.

That means that not only did the hominins living in the Jordan Valley 1.5 million years ago use different tools to survive in a different environment than those at Dmanisi, but the two groups were very different in size. It might be reasonable to call them different species.

At this height, the child at 'Ubeidya must have been one of what Barash and his colleagues call the large-bodied hominins: something like Homo erectus, which had evolved modern human limb proportions and stature by around 2 million years ago. It’s hard to say exactly which species 'Ubeidya belonged to, however.

Where to split

In part, that’s because it’s just a single vertebra. But it’s also because anthropologists don’t always agree about where, or even whether, to draw lines between species like Homo erectus, Homo ergaster, and Homo antecessor, for instance. In fact, it’s likely that the distinction matters much more to modern anthropologists than it would have to hominins living during the Pleistocene. Evolutionary change is gradual, after all. The individuals we lump into categories like Homo habilis, Homo erectus, and Homo heidelbergensis are actually just arbitrary points on an evolutionary continuum—they’re clearly different from each other, but it’s hard to say exactly where one stops and the other begins.

But if 'Ubeidya was something toward the larger, Homo erectus end of that continuum, then the hominins who lived 1.7 million years ago at Dmanisi, in what’s now Georgia, were closer to the smaller end. The Dmanisi hominins were much shorter as adults than the child at 'Ubeidya, and their bones show a mixture of relatively recently evolved traits and much more old-fashioned ones. It’s likely that the Dmanisi hominins are something in between what we know as Homo erectus and an older, smaller member of our genus like Homo habilis (which lived between 2.4 and 1.4 million years ago).

In other words, the Dmanisi hominins, with their Olduwan tools, may have been part of a wave of expansion that reached as far as modern Georgia by about 1.7 million years ago. And the 'Ubeidya hominin may have been part of a later wave, involving a different group of hominins, that reached the modern Levant by 1.5 million years ago. That version of the story could tell us something interesting about why and how the first hominins ventured beyond Africa.

Push and pull

Paleoanthropologists spend a lot of time debating why groups of hominins first ventured into places like 'Ubeidya, Georgia, and eventually China and Indonesia. In its simplest form, the debate boils down to push vs. pull: did a changing climate make the hominin homeland suddenly less hospitable, forcing groups to seek more livable environments? Or did climate change just open up new swaths of inviting territory?

If Barash and his colleagues are right, the 'Ubeidya vertebra may reveal a more complicated story.

Different groups of hominins, adapted both physically and culturally to different environments, may have left Africa at different times, and along different routes, in pursuit of their own, very different ideas of what a welcoming environment looked like. For the Dmanisi group, that may have meant arid grassland, while for 'Ubeidya and their family, it may have meant warm, humid forest.

Overall, that more complicated version of the story lines up well with what we’ve learned in recent years about the whole arc of human evolution. More than one species of Australopithecine took their first bipedal steps in Eastern and Southern Africa, while more than one hominin wandered around Europe and Asia at the same time, often interbreeding. And when the earliest members of our genus, the first human explorers, ventured out into the world, none of them did it alone.

Scientific Reports, 2022 DOI: 10.1038/s41559-021-01581-2  (About DOIs).