Scientists say two new skulls unearthed in Kenya challenge the conventional view of human evolution.  Instead of one human species evolving in succession after the other over the past two million years, the fossils reveal that two species at the dawn of human development evolved side by side. VOA's Jessica Berman reports scientists also say one fossil suggests that our nearest human ancestor may have been more primitive than previously thought.

According to the theory of evolution, there was a straight line in the development from our prehistoric ancestors to modern humans. 

A homo erectus skull (back) and homo hablis skull (foreground lower right) are shown together

The period spans two million years, beginning with homo hablis, which had a small brain and evolved, 1.5 to two million years ago, into homo erectus.  Anthropologists say erectus had a bigger brain and stood upright, looking more like modern man.

Some 250,000 years later, erectus gave way to homo sapien, or modern humans.

But in a study published this week in the journal Nature, an international team of researchers reports the discovery of two of the earliest ancestral skulls, unearthed east of Lake Turkana in Kenya, that show that homo hablis and homo erectus lived side by side for almost a 500,000 years.

Scientists say the more primitive fossil, an upper jaw of hablis, is 1.44 million years old.   The second fossil is that of an almost fully preserved erectus from 1.55 million years ago.   

Another view of the homo erectus skull

Scientists say the two species overlapped by some 500,000 years and evolved side by side instead of developing in a straight line, one right after the other.

Fred Spoor, a professor of evolutionary anatomy at University College in London, is a senior co-author of the Nature paper.  Spoor, who is now in Kenya, says the finding challenges conventional notions on human origins.

"The old idea about human evolution as a single straight line from some very primitive ape-like thing to us typically known by people perhaps from the cartoon or series of little figures from an ape figure to an upright human, that's simply not a correct picture," he said.

Until now, all of the homo erectus skulls that have been found have been approximately the same size.

But the newest erectus fossil is unexpectedly small, according to scientists, who say the skull suggests it probably belonged to a female who lived in a social setting that involved a dominant male, similar to a chimpanzee or a gorilla.

Susan Anton is an anthropologist at New York University and a study co-author.

"You know, we had really been thinking about homo erectus as something that was the first thing that sort of looked more like us in terms of body size and something more like us in terms of its rate of development," she said.  "And it turns out that all of those steps we thought that homo erectus had taken, perhaps they hadn't."

But Anton says there's no question that homo sapien eventually evolved from homo erectus.

It's just that sometimes, according to Fred Spoor, evolution is messy.

"So, it's a very bushy, as we call it, tree of evolution rather than a simple straight line," he said.  "So, it makes our lives more difficult but it makes our lives more interesting.  And it also more realistic to see us in this perspective."