The soft spot on a baby's head may be able to tell us when our ancestors first began to speak.

We have tremendously huge brains--six times bigger than the typical brain of a mammal our size. Obviously, that big size brings some fabulous benefits--consciousness, reasoning, and so on. But it has forced a drastic reorganization of the way we grow up. Most primates are born with a brain fairly close to its adult size. A macaque brain, for example, is 70% of adult size at birth. Apes, on the other hand, have bigger brains, and more of their brain growth takes place after birth. A chimpanzee is born with a brain 40% of its adult size, and by the end of its first year it has reached 80% of adult size. Humans have taken this trend to an almost absurd extreme. We are born with brains that are only 25% the size of an adult brain. By the end of our first year, our brains have reached only 50%. Even at age 10, our brains are not done growing, having reached 95% of adult size. For over a decade, in other words, we have newborn brains.

It's likely that this growth pattern evolved as a solution to a paradox of pregnancy. Brains demand huge amounts of energy. If mothers were to give birth to babies with adult-sized brains, they would have to supply their unborn children with a lot more calories in utero. Moreover, childbirth is already a tight fit that can put a mother's life in jeopardy. Expand the baby's head more, and you raise the risks even higher.

Extending the growth of the brain obviously gave us big brains, but it may have endowed us with another gift. All that growth now happened not in the dark confines of the womb, but over the course of years of childhood. Instead of floating in an aminotic sac, children run around, fall off chairs, bang on pots, and see how loud they can scream. (At least mine do.) In other words, they are experiencing what it's like to control their body in the outside world. And because their brains are still developing, they can easily make new connections to learn from these experiences. Some researchers even argue that only after the brains of our ancestors became plastic was it possible for them to begin to use language. After all, language is one of the most important things that children learn, and they do a far better job of learning it than adults do. If scientists could somehow find a marker in hominid fossils that shows how their brains grew, it might be possible to put a date on the origin of language.

That's where the soft spot comes in.

The oldest hominids that look anything like humans first emerged in Africa about 2 million years ago. They were about as tall as us, with long legs and arms, narrow rib cages, flat faces, and small teeth. The earliest of these human-like hominids are known as Homo ergaster, but they rapidly gave rise to a long-lived species called Homo erectus. H. erectus probably originated in Africa, but then burst out of the home continent and spread across Asia to Indonesia and China. The Homo erectus people who stayed behind in Africa are probably our own ancestors. The Asian H. erectus thrived until less than 100,000 years ago. They could make simple stone axes and choppers, and had brains about two-thirds the size of ours.

Paleoanthropologists have found only a single braincase of a baby Homo erectus. It was discovered in Indonesia in 1936, and has since been dated to 1.8 million years old--close to the origin of the species. While scientists have had a long time to study it, they haven't made a lot of progress. One problem is that the fossil lacks jaws or teeth, which can offer clues to the age of a hominid skull. The other problem is that the interior of the braincase was filled with rock, making it hard to chart its anatomy.

In the new issue of Nature, a team of researchers rectified this problem with the help of a CT scanner. They were able to calculate the volume of the child's brain, and then they were able to map the bones of the skull more accurately. As babies grow, the soft spot on their skull closes up and other bones are also rearranged in a predictable sequence. Chimpanzees, our closest living relatives, also close up their skulls in the same pattern, with some small differences in timing. The H. erectus baby, its skull shows, was somewhere between six and eighteen months old. Despite its tender age, the Homo erectus baby had a big brain--84% the size of adult Homo erectus brains as measured in fossil skulls.

A single battered braincase still leaves plenty of room for uncertainty, but it's still a pretty astonishing result. At a year old, this Homo erectus baby was almost finished growing its brain. It spent very little time developing its brain outside the womb, suggesting that it didn't have enough opportunity to develop the sophisticated sort of thinking that modern human children do. If that's true, then it's unlikely it could ever learn to speak. If these researchers are right, then future CT scans of younger hominid skulls should be able to track the rise of our long childhood.