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Diverse Human Body Size Emerged Earlier Than Previously Thought

Mar 28, 2015 02:15 PM EDT
Pictured: This is a cast of the 'Nariokotome Boy' (Homo ergaster) skeleton.
(Photo : Jay Stock)

Our genus comes in all different shapes and sizes, from short and fat to tall and skinny. And new research suggests that the diversity in human body size that we see today emerged earlier than scientists previously thought.

When it comes to body size, the dominant theory of human evolution is that our genus, Homo, wasn't able to migrate beyond Africa and colonize Eurasia until we progressed from small-bodied early humans to the taller, heavier and longer legged Homo erectus that we are today. This transition likely occurred approximately 1.77 million years ago, when small-bodied Homo erectus - averaging less than five foot (152cm) and under 50kg - were found living in Georgia in southern Europe.

However, the exact timing and geographic origin of the larger body size that we associate with modern humans has remained a mystery - that is, until now.

A team of researchers at the Universities of Cambridge and Tübingen in Germany has shown that the main increase in body size in fact occurred tens of thousands of years after Homo erectus left Africa - primarily in the Koobi Fora region of Kenya.

"The evolution of larger bodies and longer legs can thus no longer be assumed to be the main driving factor behind the earliest excursions of our genus to Eurasia," co-author Manuel Will from Tübingen explained in a news release.

The new findings are based on a new research method that uses tiny fragments of fossil - some as small as toes and tiny ankle bones no more than 5cm long - to estimate our earliest ancestors' height and body mass.

"What we're seeing is perhaps the beginning of a unique characteristic of our own species - the origins of diversity," said Dr. Jay Stock, co-author of the study from the University of Cambridge. "It's possible to interpret our findings as showing that there were either multiple species of early human, such as Homo habilis, Homo ergaster and Homo rudolfensis, or one highly diverse species. This fits well with recent cranial evidence for tremendous diversity among early members of the genus Homo."

The researchers compared measurements of fossils from sites in Kenya, Tanzania, South Africa, and Georgia, and discovered that there was significant regional variation in the size of early humans during the Pleistocene between 2.5 and 1.5 million years ago.

For example, some groups, like those who lived in South African caves, averaged 4.8 feet tall, whereas early humans from Kenya's Koobi Fora region would have stood at almost 6 foot - that's about the average of today´s male population in Britain.

So now scientists may have to start re-writing the history books, as it seem that humans did not evolve in body size 1.77 million years ago, but in fact after 1.7 million years ago, in the Koobi Fora region of Kenya.

"Basically every textbook on human evolution gives the perspective that one lineage of humans evolved larger bodies before spreading beyond Africa. But the evidence for this story about our origins and the dispersal out of Africa just no longer really fits," Stock concluded.

The results were published in the Journal of Human Evolution.

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