Human ancestors who diverged from chimps may have seen an increase in prenatal growth rate soon after the split, according to research from Western Washington University and the University of California, Berkeley.
The average growth rate of human fetuses is 11.6 grams per day, which is significantly more than the 8.2 grams per day growth rate of gorillas, the next fastest primate in the family of hominids.
Western Washington University’s Tesla Monson says, “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.”
In the past, researchers have relied on fossilized pelvises and the infrequent remains of newborns to examine the evolution of human gestation.
The ratio of the first and third molar teeth’s lengths was discovered by Monson and her colleagues to be a strong predictor of prenatal growth rates among primates.
By measuring the size of the molars of 608 primates, including apes and African and Asian monkeys, the researchers developed a mathematical model that could forecast prenatal growth rates.
Human Ancestors Diverged From Chimps 5 to 6 Million Years Ago
From the fossilized molar teeth of thirteen different hominid species, researchers utilized a model to predict the prenatal growth rates. This showed that the prenatal growth rates of hominids rose after our ancestors diverged from chimpanzees about five to six million years ago, becoming more comparable to those of contemporary humans than other apes about one million years ago.
Monson and her team are looking into whether specific genes might be responsible for both prenatal growth rate and the molar length ratio. She acknowledges that it might not be accurate to extrapolate prenatal growth from skeletal remnants.
She says, “Since we don’t have a time machine, we can’t directly compare our reconstructions with real values in the past.”
But over this time, hominid pelvis and brain sizes have grown along with the predicted rise in prenatal growth rates.
Monson says, “It’s really cool that our reconstructions align with so many other lines of evidence.”
At the University of Colorado Denver, Anna Warrener says, “The authors’ primary finding that human-like prenatal growth rates emerged less than [one] million years ago, in concert with major increases in brain size, is convincing.”
She adds, “Teeth are frequently found in the fossil record and would be a fantastic tool for such evaluations in the future.”