By Ian Towle, Monash University

Humanity’s family tree is long and tangled. Scientists sort our ancient relatives into three familiar groups: Homo, Australopithecus and Paranthropus.

Each of these is a “genus”, the rank just above a species. (In Homo sapiens, Homo is the genus and sapiens is the species.)

These names dominate how we talk about human origins. But new fossils, advances in genetic analysis, and more rigorous methods for figuring out who’s related to whom show these groupings don’t really reflect the true shape of our family tree.

Is it therefore time to come up with a new way to classify humans and our closely related extinct relatives? As I argue in a new paper in the American Journal of Biological Anthropology, the current system is overdue for revision.

Finding family branches

The core problem is the current groups often don’t represent real family branches – what scientists call “clades”. A clade is one common ancestor and every one of its descendants.

There’s a second issue too. Ideally, a genus should also tell us something about the behaviour, key traits, or way of life shared by all its member species.

Australopithecus (which includes the famous “Lucy” skeleton) is the clearest case. Large-scale analyses consistently find this genus isn’t a real family branch at all.

The problem traces back to how the genus was originally defined – not by shared ancestry, but by shared lifestyle or appearance. Recent fossils show traits such as brain size and gait, as well as diet and behaviour, don’t map cleanly onto the actual family tree.

Distinctive human traits are hard to find

Homo has its own problems. Traits once thought to define our genus, such as large brains, tool use and dietary shifts, and committed full-time land-dwelling bipedalism, have been undercut by more recent discoveries. Species such as Homo naledi and Homo floresiensis (the so-called “hobbit” species) pair small brains and other “primitive” features with traits typical of other Homo species.

It’s now hard to name a single feature shared by all Homo species not also found in some Paranthropus or Australopithecus species. That’s because different body parts evolve at different rates and directions, in what is called mosaic evolution. Traits also tend to evolve more than once, independently, in different closely related lineages facing similar pressures.

What’s more, key behaviours we associate with being human emerged much later than the genus itself did. The earliest members of Homo, as currently defined, include very small-brained species that may even have evolved independently from different Australopithecus lineages. The small-brained, later-surviving species mentioned above may likewise have surprisingly deep, separate roots.

Even Paranthropus, arguably the most physically distinctive of the three, known for its heavy jaws and huge molars, isn’t safe. Those features have long been read as adaptations for eating hard foods such as nuts, but evidence now suggests eastern and southern African species ate quite differently from one another, and neither relied heavily on hard foods.

Some researchers argue the group may not even be a single lineage, but rather two independent regional groups that evolved oversized teeth, meaning Paranthropus would not be a real family branch either, although most researchers today think this group is the most likely to be a genuine clade out of the three.

Taken together, there’s now little anatomical, behavioural or ecological evidence to justify keeping these three genera.

Why classification matters

This may sound like an abstract semantic question. But when genus names don’t reflect real relationships, they can obscure patterns of descent, overstate differences between groups, and reinforce outdated “primitive versus advanced” thinking. With a fast-growing fossil record and better methods for reconstructing family trees, the gap has become too large to ignore.

One solution is to fold Australopithecus and Paranthropus into a single, much larger Homo genus, which would represent a real and complete branch of the family tree. This would fix the “not a real clade” problem, align our classification with the way we treat non-human species, and better account for interbreeding between closely related lineages over the past few million years.

Changing the classification would have costs. We’d lose some intuitive distinctions researchers rely on, especially the ecological and physical uniqueness of Paranthropus and later, large-brained Homo.

Language must keep up with understanding

The fossil record seems to be increasingly showing a large, branching burst of evolution beginning around 4 million years ago, with diet, brain size, movement and tooth size all varying substantially among groups in ways that don’t necessarily map neatly onto their true relationships. A group this large and complex, shaped by migration and repeated rapid bursts of evolution in particular traits and behaviours, is best captured under a single genus.

Our everyday language is changing too. Humans were only formally recognised in recent decades as great apes – in the sense of being placed in the same family as chimps, gorillas and orangutans (Hominidae) – yet “ape” in casual usage still often excludes humans.

Similarly, “monkey” is often assumed to be a genuine evolutionary group, but isn’t. New World monkeys are more distantly related to Old World monkeys and apes than those two groups are to each other. Someone who consistently hears “monkeys, apes and humans” may reasonably draw wrong conclusions about our evolutionary relationships.

We are therefore in a transition period in how we discuss our place in the primate family tree, both in science and in everyday language. As biology increasingly embraces taxonomy that reflects genuine evolutionary relationships, perhaps it is time for our terminology, and the way we talk about ourselves, to catch up.

Ian Towle, Research Fellow in Biological Anthropology, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.