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A 3.7 million-year-old footprint discovered in Africa has been linked to an unknown human ancestor.

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The discovery shows that two bipedal hominins, rather than one, walked the Earth at the same time.

Paleontologists discovered two sets of footprints dating back 3.7 million years in 1976. The first appeared to be the oldest known evidence of upright walking among early humans, while the second appeared to be from a bear. However, scientists now believe the second pair of footprints belonged to humans as well.

Scientists believe the footprint is evidence of a previously unknown human progenitor. If this is the case, two bipedal beings – not one, as previously thought – inhabited the earth at the same time.

"These impressions show that the evolution of upright walking was more intricate and interesting than we previously assumed," Jeremy DeSilva, an anthropology professor at Dartmouth College and co-author of a new footprint research published in Nature, noted.

"At this period in our evolutionary history, there were at least two hominins walking in distinct ways, on different shaped feet, demonstrating that the learning of human-like walking was less linear than many believe."

Scientists previously believed that only one type of human existed at the period - Australopithecus afarensis, which inhabited ancient Africa 3.9 to 2.9 million years ago. The skeleton of Lucy is a well-known example of Australopithecus afarensis.

The second set of footprints, on the other hand, appear to belong to a different type of human, with a unique walking style. The footprints indicate that they swung their foot forward and landed it in front of the other, rather than walking in a straight path.

Surprisingly, scientists believe that both early people passed through the Laetoli site in northern Tanzania at the same period. They picked their way through ash turned to mud after a volcanic outburst.

Ellison McNutt, a biological anthropologist at Ohio University's Heritage College of Osteopathic Medicine, speculated that the two early people may have caught a glimpse of each other at some point.

She hypothesised that the unknown owner of the second pair of footprints "looked up across the countryside and saw an Australopithecus afarensis travelling somewhere else."

"It's pretty cool that we may have two hominin species, at least, living in the same place," McNutt remarked.

The volcano erupted again after this possible contact. The footprints at Laetoli were preserved for millions of years by a new layer of ash. However, it took considerably longer for academics to decipher their importance.

Paleontologists discovered Australopithecus afarensis prints at Laetoli site G in 1976. They determined that they were the earliest known footprints left by bipedal humans. They also discovered tracks at Laetoli site A, but they couldn't tell if they were left by a bear or a human.

"Neither explanation convinced scientists," said Stephanie Melillo, a paleoanthropologist at the Max Plank Institute for Evolutionary Anthropology. "In the end, the site A prints were easier to forget than explain."

DeSilva and his team did not set out to uncover and investigate the footprints at Laetoli site A until recently. After digging the site again, he and his team observed how bears walked and discovered that they only walk on two feet 1% of the time.

As a result, the footprints at Laetoli site A are more likely to have belonged to an early species of man rather than an ancient bear.

Melillo explained that the "site A footprints are unlike those of any other hominid," and that DeSilva's team conducted a "convincing job" of establishing the prints didn't belong to a bear.

"The footprints themselves are unusually wide and short, and the feet responsible for their production may have had a big toe capable of thumb-like grabbing, comparable to apes' big toe."

And, if they actually belong to early man, the prints call into question previously held beliefs about the path of early human history.

"If there is a second species, it shows that Australopithecus afarensis and something else existed at the same time and in the same region," Meillo added.

For the time being, this enigmatic human ancestor remains a mystery. Despite DeSilva's claim that it was just three feet tall, only fossil evidence can provide light on what this early human looked like.