How and why, roughly 2 million years ago, early human ancestors evolved large brains and began fashioning relatively advanced stone tools, is one of the great mysteries of evolution. Some researchers argue these changes were brought about by the invention of cooking. They point out that our bite weakened around the same time as our larger brains evolved, and that it takes less energy to absorb nutrients from cooked food. As a result, once they had mastered the art, early chefs could invest less in their digestive systems and thus invest the resulting energy savings in building larger brains capable of complex thought. There is, however, a problem with the cooking theory. Most archaeologists (考古学家) believe the evidence of controlled fire stretches back no more than 790,000 years.
Roger Summons of the Massachusetts Institute of Technology has a solution. Together with his team, he analyzed 1.7 million-year-old sandstones that formed in an ancient river at Olduvai Gorge in Tanzania. The region is famous for the large number of human fossils(化石)that have been discovered there, alongside an impressive assembly of stone tools. The sandstones themselves have previously yielded some of the world's earliest complex hand axes -- large tear-drop-shaped stone tools that are associated with Homo erectus(直立人). Creating an axe by repeatedly knocking thin pieces off a raw stone in order to create two sharp cutting edges requires a significant amount of planning. Their appearance is therefore thought to mark an important moment in intellectual evolution. Trapped inside the Olduvai sandstones, the researchers found distinctive but unusual biological molecules（分子）that are often interpreted as biomarkers for heat-tolerant bacteria. Some of these live in water between 85℃ and 95℃. The molecules' presence suggests that an ancient river within the Gorge was once fed by one or more hot springs.
Dr. Summons and his colleagues say the hot springs would have provided a convenient "pre-fire" means of cooking food. In New Zealand, the Maori have traditionally cooked food in hot springs, either by lowering it into the boiling water or by digging a hole in the hot earth. Similar methods exist in Japan and Iceland, so it is plausible, if difficult to prove, that early humans might have used hot springs to cook meat and roots. Richard Wrangham, who devised the cooking theory, is fascinated by the idea. Nonetheless, fire would have offered a distinct advantage to humans, once they had mastered the art of controlling it since, unlike a hot spring, it is a transportable resource.
