To a paleontologist, early humans and their relatives are a bit like paper dolls. Instead of hats or dresses, the interchangeable pieces are an array of fossilized body parts, ranging from apelike to almost human.
Scientists used to think that the dress-up rules were fixed, that “you only found those pants with that top, and you only found those pants and that top with these shoes,” says Bernard Wood of George Washington University. If you unearthed an ankle bone bearing some resemblance to a human ankle, for instance, you would assume that the owner of that ankle had a similarly humanlike heel, and a certain proficiency in walking upright.
But it turns out that human evolution is about as unpredictable as a six-year old experimenting with paper-doll fashion. From a site in South Africa, Lee Berger of the University of Witwatersrand has excavated some unusually well-preserved fossils with both advanced and primitive body parts, mixed and matched in surprising ways. A humanlike ankle is paired with an apelike heel, for example. The two million-year old mosaic foot seems to have allowed its owner to walk upright, yet with a mechanism quite unlike our own.
The South African fossils suggest that “human” features were not confined to the lineage that gave rise to humans. Rather, humanlike traits seem to have evolved in multiple branches of our family tree. All of this variability makes it hard for us to tell our direct ancestors apart from our extinct cousins as we study the fragmentary fossil record. Apparently, we need to look at many more fossils of similar completeness—comparing entire outfits, rather than just pants or shoes—before we can begin to find trends in evolutionary fashion.