Our ancestors’ prenatal growth sped up after we split from chimps

Early humans evolved a faster fetal growth rate than other apes about a million years ago, suggesting it could have played a role in the evolution of our species

By Carissa Wong

A series of hominid craniums (clockwise from the left): juvenile Australopithecus, juvenile chimpanzee, adult chimpanzee, adult Australopithecus, adult Homo erectus

Tesla Monson

High prenatal growth rates found in modern people may have first evolved in ancient hominids less than a million years ago, according to estimates based on fossil teeth.

Human fetuses grow by around 11.6 grams per day on average – considerably faster than the fetuses of gorillas, the next fastest ape in the hominid family, with a rate of 8.2 grams per day.

“We found that human-like gestation [may have] preceded the evolution of the [modern human] species – around 200,000 to 300,000 years ago – and may in fact be a critical factor that led to our evolution, particularly our large brains,” says Tesla Monson at Western Washington University.

Previously, researchers studying the evolution of human gestation have relied on fossilised pelvises and the rare remains of infants.

Monson and her colleagues found that across primates, prenatal growth rates are closely correlated with the ratio of the lengths of the first and third molar teeth.

The researchers built a mathematical model that could predict prenatal growth rates from the size of molars from 608 primates, including apes and African and Asian monkeys.

They then used the model to predict the prenatal growth rates of 13 hominid species from their fossil molar teeth. This revealed that hominid prenatal growth rates increased after our lineage split from chimpanzees around 5 to 6 million years ago, becoming more similar to those of modern humans than other apes around 1 million years ago.

Monson is unsure why prenatal growth rate and the molar length ratio may be related, but she and her team are investigating whether certain genes might control both. She acknowledges that extrapolating prenatal growth from skeletal remains may not be reliable. “Since we don’t have a time machine, we can’t directly compare our reconstructions with real values in the past,” she says.

However, the estimated rise in prenatal growth rates over this period coincides with increases in pelvis size and brain size among hominids. “It’s really cool that our reconstructions align with so many other lines of evidence,” says Monson.

“The authors’ primary finding that human-like prenatal growth rates emerged less than 1 million years ago, in concert with major increases in brain size, is convincing,” says Anna Warrener at the University of Colorado Denver.

“Teeth are frequently found in the fossil record and would be a fantastic tool for such evaluations in the future,” she says.

“The study is of great importance. It is incredibly difficult to access information about fetal growth rates from skeletal remains due to poor preservation. The authors have opened up new ways of overcoming this obstacle,” says Patrick Mahoney at the University of Kent, UK.

Journal reference: PNAS, DOI: 10.1073/pnas.2200689119

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