VOA慢速英语--人类早期头骨的年龄估算有了惊人的发现（在线收听）

New Age Estimate for Early Human Skull Produces Surprising Discovery

Scientists say a new age estimate for the skull of an early human is raising questions about modern human ancestry.

The fossilized skull was considered a major discovery when it was found in Zambia in 1921. It was the first fossil of an extinct human species to be discovered in Africa.

The skull was named after the area in which it was discovered: Broken Hill. Researchers say a new examination of the fossil suggests that it is much younger than previously thought.

The fossil was difficult for scientists to date. That is because the skull was recovered in a mine that later became completely destroyed from mining operations.

Researchers now say that two new complex dating methods have estimated the Broken Hill skull to be about 299,000 years old. Past scientific examinations had estimated the fossil was likely about 500,000 years old.

The team of researchers said their examination efforts involved years of work that included direct dating of the skull itself as well as other human and non-human materials found near the discovery area.

Rainer Gruen of Australia's Griffith University led the examination efforts. He was also the lead writer of a study on the process, which recently appeared in the publication Nature.

Gruen said in a statement that the new age estimate changes earlier opinions on both "the tempo and mode" of modern human ancestry.

Gruen and his team believe the findings show that human evolution in Africa around 300,000 years ago "was a much more complex process" than other evidence has suggested.

Scientists now believe the process involved the co-existence of several human lineages. Instead of linear evolution, in which each new species replaced the old one, Africa may have been a place where different human species mated with each other.

The new estimate means the species represented by the skull was unlikely to have been a direct ancestor of Homo sapiens, the species of human that exists today. Our species first appeared in Africa more than 300,000 years ago before spreading worldwide.

Scientists first thought the skull belonged to a new species they named Homo rhodesiensis. But most scientists now believe it is part of the Homo heidelbergensis group. This species is thought to have appeared about 600,000 years ago in parts of Africa and Europe.

Chris Stringer is an anthropologist with the Natural History Museum in London. He told the Reuters news agency the latest research suggests "that the facial shape of Homo heidelbergensis fossils does not fit an ancestral pattern for our species."

Stringer said the age of the fossil suggests that at least three human species lived in Africa around 300,000 years ago. Homo sapiens, he said, were likely present in places like Morocco and Ethiopia. The Homo heidelbergensis is thought to have lived in south-central Africa. The third is called Homo naledi, and was recently discovered in South Africa.

"We already knew that Eurasia contained diverse human lineages about 300,000 years ago," Stringer said. "Now, the same applies to Africa."

When the skull was discovered in 1921, it provided the first evidence of a prediction that British naturalist Charles Darwin made 50 years earlier. Darwin theorized that Africa was the place where humans broke off from other animals, because African apes are our closest living relatives. Prehistoric human fossils until that time had all been found in Europe and Asia.

I'm Bryan Lynn.

Words in This Story

skull – n. the part of the head that is made of bone and protects the brain

fossil – n. part of an animal or plant from thousands of years ago, preserved in rock

extinct – adj. no longer living or existing

tempo – n. the speed at which something happens

mode – n. a way of doing something

evolution – n. the way in which living things gradually change and develop over millions of years

species – n. a set of plants or animals in which the members have similar characteristics to each other

anthropologist – n. a person who studies human development and society or different societies

pattern – n. a particular way something is done, is organized or happens

diverse – adj. including many different kinds