Rare Homo Naledi Juvenile Fossils Provide Clues Into How Early Humans Aged

Thursday, April 16, 2020

Homo naledi inhabited Earth at about the same time as Homo sapiens, the early ancestors of modern humans (Credit: John Hawks/University of the Witwatersrand)

Over the years, paleontologists have been able to uncover many mysteries about human ancestors from unearthed skeletal remains. However, not much is known about their development and growth. because most hominin fossils are those of adults, and remains of developmentally young hominins are uncommon. Now, a perfectly-preserved partial skeleton of a Homo Naledi juvenile, who lived on Earth between 335,000 and 226,000 years ago, ​ is providing insights into how our ancient relatives may have aged.

The rare remains of the young individual, dubbed DH7 (Dinaledi Hominin 7), were among a treasure trove of bones found in South Africa's Dinaledi Chamber of the Rising Star cave system in 2013 and 2014 by Dr. Debra Bolter and her colleagues at the University of the Witwatersrand. Prior to leaving the area, the scientists carefully documented the over 1,800 fossils so they could accurately reconstruct each skeleton once they returned home.

An artist's illustration of Dinaledi Chamber in South Africa where remains of 15 H. naledi individuals, including a juvenile, were discovered (Paul H. G. M. Dirks et al / CC -BY-SA 4.0/Creative commons.org)

Over the years, Bolter and her team have determined that the remains, which include 1,550 bones and teeth, belonged to at least 15 H. naledi individuals. While all were interesting, DH7's perfectly-preserved arm and leg bones and partial jaw, were particularly so, because they revealed a blend of maturation patterns observed in modern humans and earlier hominins.

A detailed study of the remains, published in the journal PLOS ONE on April 1, 2020, revealed that while the young H. naledi had curved hand bones like our older, faster-developing ancestors, its feet and ankles looked very similar to those of slower-growing modern humans. The anatomical mishmash becomes more confusing when scientists consider that H. naledi's brain was much smaller than our modern-day human ancestors, H. sapiens, and more in line with our primitive ancestors like the H. erectus or A. sediba. However, it was more complex and developed than that of the earlier hominins.

Homo naledi juvenile remains offers clues to how our ancestors grew up. Credit: Bolter et al. PLOS ONE 2020 (CC BY)

The scientists estimate that DH7 was between 8 and 15 years old at the time of its demise. However, they are not sure of its exact age. Its small brain would typically indicate that DH7 matured as fast as the earlier hominins and was between 8 and 11 years old when it died. However, its human-like features and complex brain could mean that DH7 aged more slowly, like modern humans and Neanderthals, making it between 11 and 15 years old. "You're beginning to see that maybe H. naledi is breaking all the rules," says study co-leader Lee Berger.

Scientists may be able to determine the juvenile H. naledi's age by its teeth, but the process poses risks (Credit: Debra Bolter et al/ University of the Witwatersrand)

The researchers could determine DH7's age more accurately by counting the faint lines left behind by the accumulating enamel on its developing teeth. However, in order to do so, they would have to cut into the teeth or expose them to powerful x-rays. While the former would destroy the precious fossils, the latter could kill any preserved proteins that may help unveil more secrets about H. naledi's connection to other hominins, including modern-day humans. "We just have to be careful that what we do to get one piece doesn't destroy an attempt to get any other kind of data," Bolter says.

The paleontologists plan to continue studying the juvenile H. naledi to get more insights into how it aged and, more importantly, where the young individual fits into the evolution of the growth and development of modern-day humans.

Resources: www.nationalgeographic.com, www.sciencedaily.com