Scientists have discovered stone tools that date back 3.3 million years, and the finding could rewrite our understanding of human evolution, reports

Previously, the oldest stone tools ever found were believed to have been fashioned by Homo habilis, the first species included in the Homo genus, somewhere between 1.5 and 2.8 million years ago. The age of the newly discovered tools pushes that date back at least 700,000 years, which is before the Homo genus had even evolved. That means the first creature to ever bang two stones together to create a new technology may not have been a direct human ancestor after all. It's a surprising finding, and opens the door to all sorts of new questions about early hominin evolution. 

The tools "shed light on an unexpected and previously unknown period of hominin behavior and can tell us a lot about cognitive development in our ancestors that we can't understand from fossils alone," said lead author Sonia Harmand.

"Hominins" are what scientists call members of the human clade that evolved after the split from the chimpanzees. Our world today contains only one species of hominin: us. But the world that our earliest ancestors inhabited was quite a bit more diverse, with several evolutionary branches that included a number of species that are not necessarily our direct ancestors.

Ancient hominins included in the Homo genus are the ones with the closest relation to modern humans (we are, after all, Homo sapiens). It has long been believed that the fashioning of stone tools by knapping two stones together was exclusively a Homo technology, but this new finding challenges everything. 

So if there were no hominins in the Homo genus around when these oldest of tools were fashioned, then who or what created them? Scientists still aren't sure, but the leading candidate is a hominin called Kenyanthropus platytops. A K. platytops skull was found in 1999 only about a kilometer from the tool site, and it also dated to around 3.3 million years old. 

Exactly how K. platytops relates to modern humans is still a contentious issue among anthropologists. There is even a question about whether K. platytops deserves its own genus; there are a number of experts who believe it should be included in the genus Australopithecus, a group of hominins that includes the famous "Lucy." Either way, the fact that such sophisticated stone tools were being created so early in hominin evolution is further indication that the evolutionary puzzle still has many missing pieces.

The finding may also rewrite our theories about why our early ancestors first began making stone tools in the first place. Conventional thinking is that hominins started knapping in order to fashion sharper stones to better slice meat off of animal carcasses, but the size and marking of the newly discovered stones suggests otherwise. It's possible the tools were instead first being used for breaking open nuts or tubers, or perhaps for bashing open dead logs to get at insects inside. If that's the case, then early hominins may not have been the meat-eaters that some theorists have suggested.

"I realized when you [figure out] these things, you don't solve anything, you just open up new questions," said geologist Chris Lepre, co-author of the study. "I get excited, then realize there's a lot more work to do."

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