Lucy could join the same genus as modern humans in a proposed shake-up of the human family tree

Big brains and stone tools no longer mark out the genus Homo, and a new proposal folds 5 million years of ancestors into this one group.

Lucy is currently classified in the genus Australopithecus, the small-brained half of the human family tree. Paleontologists do not consider her one of the big-brained, tool-making species in the genus Homo.

However, after decades of new fossils, that split fits the evidence so poorly that one researcher wants it gone.

His proposal would gather every close human relative of the past four to five million years into a single, expanded genus Homo. Lucy would become Homo afarensis.

There's no new bone in it. The question is whether the labels still match what the fossils show.

Ian Towle is a research fellow at the Monash Biomedicine Discovery Institute in Melbourne. He worked through the fossil record, the genetic evidence from living primates, and decades of published family trees.

In an interview with Earth.com, he started with the museum label. "Lucy would become Homo afarensis rather than Australopithecus afarensis," Towle said. "Importantly, this is a proposal rather than an established change in taxonomy."

Australopithecus is not a single branch

For 50 years, biology has moved toward naming only real branches of the tree of life. A branch means one ancestor plus every descendant it produced, with none left out. Biologists call that a clade.

Australopithecus isn't one, and the family trees published in recent years keep showing why.

"Different species currently placed within that genus are often more closely related to Homo or Paranthropus than they are to one another," Towle said.

Paranthropus is the group with the massive jaws and grinding molars, best known from Paranthropus robustus in South Africa. If some Australopithecus species are closer to them and others are closer to us, one name is covering pieces of two branches.

None of this is new. Researchers have described the problem since the early 1990s and kept using the name anyway. This is partly because splitting the genus up would only make a scatter of new names. Familiarity isn't a good enough reason to keep it, Towle said.

Current names reflect discovery order

The reasoning that put Lucy where she is was written down plainly in 1983. Her species, Australopithecus afarensis, hadn't grown a bigger brain, and its members clearly walked upright, so they went into the genus Australopithecus instead of Homo. Brain size drew the line.

Asked what surprised him most while working through the fossil evidence, Towle pointed at that history rather than at the bones.

"What surprised me most was how much of our current taxonomy is shaped by the order in which fossils were discovered and named," he told Earth.com.

The name is formally attached to one species, Australopithecus africanus, from South Africa. Holding the rest of the group around it has gotten harder with every new find.

"If we were to start from scratch today, we would therefore almost certainly use a very different system for classifying and discussing human evolution," he added.

Small brains broke the Homo genus definition

Homo was supposed to be the easy genus to spot. A large brain, stone tools, a body fully committed to walking on the ground, and teeth relieved of the toughest grinding.

Then came fossils that wouldn't cooperate. Homo naledi, from a South African cave, has a brain in the Australopithecus range, plus hands and shoulders built for climbing. Homo floresiensis, from Indonesia, stayed small in the same awkward way.

Both are classified as Homo. Neither meets most of the criteria.

"Large brains, fully committed terrestrial bipedalism and tool use don't form a neat package, they appear at different times and in different combinations," he said.

The blurring runs the other way too. Australopithecus garhi has bigger teeth than Paranthropus boisei, the heaviest chewer in that group, and it is still an Australopithecus. Meanwhile the oldest fossils assigned to Homo keep getting older.

Towle said the field has to apply one rule. If Australopithecus stays a stage of evolution rather than a branch, then the small-brained members of Homo have to be judged the same way.

Primate genera usually run much deeper

Homo, as used today, covers roughly the last 2 to 3 million years of hominin evolution. Across the rest of the primates, a genus usually spans 5 to 10 million. Ours is a very young one.

Towle said a genus that young may be too unstable to work as a category, and his comparison comes from living monkeys.

Baboons and their close relatives spread across Africa over roughly the same stretch in which Australopithecus, Paranthropus, and Homo were diversifying. Mangabeys were treated as one group for years. Researchers later split them in two, and molecular studies confirmed the halves belong on separate branches.

In 2005, researchers described a new monkey and placed it with the mangabeys. A year later, genetic research showed that it was closer to baboons, so scientists gave it a genus of its own.

Researchers have recorded hybrids between geladas and baboons, and between a mangabey and an olive baboon. Both crossings run between genera. Boundaries drawn that recently tend not to hold.

Where the new Homo genus starts

The human record has a further problem. For most of these fossils, no genetic data exists at all. Collapsing three genera into one will raise questions about where the new grouping starts.

Towle would draw it at Australopithecus anamensis, the earliest branch in the group.

In most recent analyses, A. anamensis falls outside everything else: the other Australopithecus species, all of Paranthropus, and all of Homo. Agreement that broad is rare in this field, and most researchers already accept that branch point.

The older candidates stay outside. This is because there is little evidence that Sahelanthropus, Orrorin, and Ardipithecus - the disputed fossils sometimes called the earliest members of the human line - belong inside.

Kenyanthropus, named in 2001 for material that fit nowhere else, would be folded in.

Some information would be lost

A genus name carries information, and merging three of them would lose some. Homo and Paranthropus lived alongside each other, were built differently, and differed in what they ate. One shared name stops marking any of that.

Towle raised that objection himself, and said it has enough merit to keep his proposal from gaining ground.

The paper is also too new for a formal reply. Asked by Earth.com how colleagues had responded, Towle said his reviewers backed the questions without necessarily backing his answer.

He expects many others to land in the same place: the problem is hard to ignore, and no simple fix is on offer.

The proposal needs backing first

Where several of these fossils actually belong is still unsettled, and Homo naledi and Homo floresiensis shift positions from one analysis to the next.

Some analyses also raise the possibility that the earliest Homo species came from different Australopithecus lineages, which would leave Homo failing the very same test.

"I'm not claiming to have solved human taxonomy, I'm trying to reopen a discussion that I think the fossil evidence, together with what we are learning from living primates, makes increasingly difficult to avoid," Towle said.

Other researchers have to start using the new names before anything changes. For now, the label under Lucy still reads Australopithecus afarensis.

The full study was published in the American Journal of Biological Anthropology.

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