Happy Birthday, Zinjanthropus

Celebration / by Sean B. Carroll /

The Leakeys’ discovery of the “Nutcracker Man” 50 years ago electrified the scientific community and refocused thinking about the origin of humans back on Africa.

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By 1951, two decades had passed without any trace of ancient human remains. Mary and Louis returned to Olduvai determined to find them. One challenge was to decide where to excavate. The gorge was very large, with more than 40 kilometers of rock exposures. They concentrated on what was called Bed II, a lower and therefore older site. Through the 1950s they catalogued thousands of items, including many very large mammal bones, but still no hominids.

In July 1959 they moved to Bed I, the lowest and oldest deposits at Olduvai. On the morning of July 17, with Louis sick in bed with a fever, Mary went prospecting, accompanied as usual by her Dalmatians. On a hillside, projecting from below the surface, she saw a scrap of bone that looked like a piece of a skull. She brushed away some sediment and saw parts of two large teeth in the upper jaw—they were definitely hominid.

She bolted back to camp in her Land Rover and called out to Louis, “I’ve got him! I’ve got him! I’ve got him!”

“Got what? Are you hurt?” Louis asked.

“Him, the man! Our man!” Mary replied.

They both raced back to the site and Louis saw at once that Mary’s finding was indeed a hominid, and a lot of the skull was there.

Well, it was there, but in 400 pieces. Once she freed them from the sediment, Mary carefully assembled the bits into a hominid that she affectionately referred to as her “Dear Boy.” Her reconstruction revealed a small-brained creature—the skull was about 500 cubic centimeters in volume, compared to 1400cc in modern humans—with such huge molars for heavy chewing that the fossil also earned the moniker “Nutcracker Man.” Louis had the responsibility for formal naming, and he settled on Zinjanthropus boisei¬, the first word meaning “man from East Africa,” and the second a hat tip to their benefactor, London businessman Charles Boise.

Everyone wanted to know the age of “Zinj.” Louis, relying on then current thinking about geological ages, estimated it was more than 600,000 years old. But newly developed potassium-argon dating methods soon revealed that the ash bed above where “Zinj” was found was a stunning 1.75 million years old, give or take a quarter million years—far older than Louis or anyone had imagined. Moreover, because “Zinj” was found in a tool-bearing layer, it was clear that toolmaking and toolmakers had been around Olduvai for a very long time.

Finding “Zinj” was not the end but just the beginning. The National Geographic Society stepped in with the largest grant the Leakeys had ever received. Flush with funding, the very next year (1960) two other toolmakers, Homo habilis and Homo erectus were discovered. The hominid family tree was branching at Olduvai.

Where “Zinj” sat in that tree was not so clear. Louis had given him his own genus because it was evident that he was not in the direct line to Homo. He was soon reclassified as a relative of the Australopithecus fossils that had been found previously in South Africa, but were not generally accepted as hominids until the Leakeys’ find. “Zinj” is known today as Paranthropus boisei.

But his bloodline is not what matters. Thanks to the revolution “Zinj” started, over the past 50 years we have learned that we are the descendants of skilled toolmakers and explorers who left Africa just 60,000 years ago, populated five continents, and then, with some new tools, built a spaceship that went to the Moon and back.

Happy birthday, “Zinj,” and happy anniversary to all of us.

Sean B. Carroll is an evolutionary biologist at the University of Wisconsin. His newest book, Remarkable Creatures: Epic Adventures in the Search for the Origins of Species (Houghton Mifflin Harcourt), chronicles the experiences and discoveries of intrepid naturalists who developed and advanced the theory of evolution.

Originally published July 25, 2009

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