A Quick Look at Humans' Thorny Shrub of a Family Tree

Human evolution, as a topic, is of course an extraordinarily contentious subject of debate, even among scientists who agree on the basics of the primate-to-human evolutionary ladder.

The family tree for Homo sapiens actually looks far more like a dense, thorny rosebush than a stately oak, and it’s filled with the kind of relatives most of us wouldn’t want to see around the Thanksgiving dinner table (or maybe we would — might make for a lively holiday). And because there is only one member of the genus Homo currently trotting around this planet — that would be us — the family tree is also filled with many long-lost relatives who wandered off into the evolutionary savannah never to be seen again.

For example, there’s the earliest hominins, collectively referred to as the Ardipithecus group, who were around from about 6.5 million years ago until about 4 million years ago. They gave way in large part to the australopithecines like “Lucy,” who hung around from about 4.5 million years ago until 2.2 million years from the present. Lucy and her crew were important because from them sprang two new groups: Paranthropus, a mostly herbivorous group of grub and nut eaters, and Homo. Homo habilis, our earliest direct progenitor, then split into two groups, one evolving into Homo erectus, which went extinct about 1.2 million years ago, and one eventually evolving into Homo sapiens.

Todd Koetje, an associate professor of Anthropology at Western, says that the way scientists view these groups is dynamic and ever changing, in part because the sample size is tiny.

“Homo erectus, for example, was around for about 1.5 million years, but we have only about 125 specimens of them worldwide,” he says.

Koetje says that the easiest way to think of all these ancestral groups is to put them into boxes. Get a big box for all the australopithecines, for example. Inside are smaller boxes for individual species such as Australopithecus africanus and Australopithecus afarensis. Samples found in the field get put into these proverbial boxes.

Or do they? What happens when you find something that looks sort of like one thing and sort of like another?

“You make a new box, and give it a new name,” he says. “And hopefully you or other folks will find more like it. Because here’s the reality; Australopithecus didn’t one day decide it was now a Homo habilis. The line isn’t black or white.”

These are the blurry edges of evolutionary science. In some ways, it’s all one flowing timeline; in others, it’s a maze of dead ends with very few clear paths to the exit marked “modern humans” — but that’s part of the fascination for scientists like Koetje.

“We are forced to open and label new boxes all the time, because researchers keep finding new specimens that they don’t yet understand,” he says. “And because of the dynamic nature of the biological and evolutionary processes, it will always be that way.”