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Why it might be time to rethink the human family tree
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.
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 behavior, 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 behavior, don’t map cleanly onto the actual family tree.
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.