Homo habilis is the oldest named hominid. But is it human?

For 60 years, the oldest known human species was also one of the most mysterious. Homo habilis was added to the family tree in 1964. But for a long time, it was unclear what this ancient species, which lived from about 2.4 million to 1.65 million years ago, looked like.

That’s because until recently, only three extremely incomplete fossilized skeletons had been unearthed.

And in January, researchers described a fourth, more complete skeleton, revealing that Homo habilis has a very different anatomy than ours. This discovery has some researchers asking big questions. Were the oldest known human ancestors not humans after all?

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“As we discover more fossils, we’ve expanded the definition of the genus Homo,” Bernard Wood, a paleoanthropologist at George Washington University in Washington, DC, told Live Science. “Maybe I stretched it out too much this time.”

Obviously, our species, Homo sapiens, belongs to the genus Homo. We also know that our closest living relatives, chimpanzees and bonobos, do not. This means that the genus Homo evolved at some point after the human evolutionary lineage, including humans and extinct relatives, split from the chimpanzee lineage, an event that occurred more than 5 million years ago. So when exactly did the human species evolve?

One approach would be to argue that it dates back to a split with the chimpanzee lineage. But the first creatures to appear after the split are not very similar to us. These include species like Australopithecus afarensis, which have long ape-like arms and relatively small brains. This species existed in Africa about 3.9 to 2.9 million years ago and includes the famous Lucy skeleton. Few researchers consider Lucy to be human.

However, most anthropologists have historically considered H. habilis to be a member of the genus Homo.

Few skeletons

The first highly incomplete Homo habilis skeleton was discovered in Tanzania in the 1960s. The 1.75-million-year-old specimen contained skull fragments, from which they could be estimated to belong to an individual whose brain was about 45% the size of an average living person. This may sound small, but it was significantly larger than the average Australopithecine brain (about 35% the size of a human brain). This evidence led to the skeleton being placed in the genus Homo and given the name Homo habilis, meaning dexterous or skilled man, a decision that most researchers accepted.

But the skeleton of Homo habilis, described in 2026, complicates matters. The 2 million-year-old skeleton was discovered in Kenya, about 500 miles (800 kilometers) north of where the first Homo habilis remains were excavated. Like the first skeleton, the Kenyan skeleton is far from complete. But the surviving bones are the best look yet at Homo habilis’ arms, said study co-author Carrie Mongul, a paleoanthropologist at Stony Brook University in New York. The problem is that its arms are different from ours. Instead, they are long and ape-like, resembling the arms of australopithecine relatives like Lucy.

“They are very australopith-like,” paleoanthropologist Ian Tattersall of the American Museum of Natural History in New York City told Live Science. In a paper published earlier this year, Dr Tattersall argued that these ape-like arms clearly indicate that Homo habilis is not a member of the genus Homo.

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He is not the first to make this suggestion. In 1999, Wood and his colleague Mark Collard, an archaeologist at Canada’s Simon Fraser University, argued that Homo habilis was not a member of the genus Homo. A second and third H. habilis skeleton had been discovered by then, and although it was very incomplete, they suggested that the species had different limb proportions than ours, which is also supported by the fourth skeleton.

Wood and Collard proposed moving the species into the same genus as Lucy, which would mean renaming Homo habilis to Australopithecus habilis. Tattersall doesn’t think that’s a good solution, since the species had brain size and teeth similar to humans. He thinks habilis should be included in a separate genus, but he hasn’t come up with a name yet.

another approach

Meanwhile, other researchers suspect that both Wood and Tattersall are wrong.

Despite its prowess, they believe there is no need to change the name of H. habilis. “Ape-like limb proportions don’t necessarily tell us much,” Carol Ward, an anthropologist at the University of Missouri, told Live Science. This is because of the way most evolutionary scientists define species and genera.

We know that our earliest ancestors lived shortly after the chimpanzee lineage diverged, and spent much of their time climbing trees, where long ape-like arms would have helped. They gradually adapted to spend more time walking on land and eventually evolved into humans.

These bipedal ancestors probably no longer needed the long arms of apes. But importantly, long arms are almost certainly not an impediment to survival, Ward said. In such a situation, even the first species of the genus Homo may have retained the long arms of their ancestors. This is because there was no strong evolutionary pressure to shorten the arms. It’s still not entirely clear why the arms eventually shrunk, but some researchers believe that shorter arms may have provided subtle advantages when running or using tools. This suggests that there was a weak evolutionary pressure for shorter arms, meaning that the arms shrank, but at a relatively slow rate.

There is a broader point here. “We like to think that there’s been a huge change in the Homo race and that we’re different from everything that came before,” Ward said. “But this Homo habilis skeleton supports the idea that there was probably a more gradual transition from australopiths to humans.”

The idea highlights thorny questions that scientists are still grappling with.

Evolution is so complex that it is surprisingly difficult to divide organisms into distinct groups, such as species. This is one reason why there are currently dozens of different ways to define species, and there is intense debate about which is best. Defining the genus proved equally difficult. That means there’s really no consensus on what a genus is, Wood said.

In other words, researchers will probably continue to debate whether H. habilis belongs to the genus Homo, simply because they still can’t fully agree on what the genus actually is.

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