Rate of Human Development First Slowed Down 1.77 million Years Ago

The pace of human physical development is slower than that of other primates, meaning that it takes us longer to move from childhood through adolescence to adulthood. Scientists have long believed that this is related somehow to the complex nature and large size of the human brain, which takes more time to mature in harmony with the human body.

There are questions about how early the ancestors of modern humans (Homo sapiens) started physically maturing at a delayed rate, in comparison to their primate cousins. In a new study just published in the journal Nature, a team of European scientists presents evidence that suggests delayed growth in the human evolutionary line first appeared in approximately two million BC, or at least 500,000 years earlier than previously believed.

“We show that the first evolutionary steps towards an extended growth phase occurred in the genus Homo at least 1.77 million years ago, before any substantial increase in brain size,” the study authors wrote in their Nature article.

The intriguing new discovery is significant, because it does not support the idea that human physical growth slowed in conjunction with increased development in the brain, at least not in the earliest stage of the process.

Signs of Delayed Childhood at Dmanisi

The scientists involved in the newly published study studied ancient skeletal remains recovered in eastern Europe, specifically in central Georgia at a prehistoric archaeological site near the city of Dmanisi. In this case it was the fossilized teeth of an ancient hominin child that produced the evidence of delayed growth in archaic humans, which the scientists obtained using sophisticated x-ray imaging technology to count growth lines on the 1.77-million-year-old specimen’s molars.

The Dmanisi specimen was found in 2001, and its remains included a skull and jaw with several teeth still in place. Paleoanthropologists Christoph Zollikofer and Marcia Ponce de León from the University of Zürich and paleontologist Paul Tafforeau from the European Synchrotron Radiation Facility used synchrotron power to secure the high-resolution x-ray images of the dental growth lines, which showed the child had died between the ages of 11 and 12.

Dentition of Dmanisi skull D2700/D2735. a–c, Computed tomography (CT)-based visualization of the skull and associated teeth showing right lateral (a), frontal (b) and left lateral (c) radiographic projections. d,f, Occlusal views of reconstructed mandibular (d) and maxillary (f) dentition (green, highresolution PPC-SRµCT of isolated teeth used for microstructural analyses; orange, medium-resolution PPC-SRµCT of teeth in place, not included in microstructural analyses; red, low-resolution scans (medical CT, not included in analyses); light blue, missing teeth mirrored from preserved antimeres; dark blue, missing mandibular first incisors, reconstructed from second incisors). e, Frontal view of the skull. Scale bar, 5 cm ((Vincent Beyrand and Paul Tafforeau, European Synchrotron Radiation Facility, Grenoble, France/Nature)

The researchers analyzed stress lines on the ancient teeth, caused by illness or a lack of food, to see how the various teeth in the child’s mouth grew in relation to each other.  By doing this they were able to determine that the Dmanisi child kept its baby teeth longer than most primates during the first five years of its life, which is the same pattern observed in modern humans. But the child then experienced more rapid growth in the molars between ages six and 11, this time matching the growth patterns of a modern chimpanzee.

What happened during the first five years was most significant, because it represents the oldest example of a slowdown in the rate of hominin maturation ever found in the fossil record.  

Notably, this particular hominin species possessed a brain that was similar in size to that of a chimpanzee. While the brain’s developmental pace may have been accelerating by this time, its size and complexity were still far from that of modern humans.

“One of the main questions in paleoanthropology is to understand when this pattern of slow development evolves in [our genus] Homo,” explained Alessia Nava, a bioarchaeologist from Sapienza University in Rome who was not involved in the new study, in an interview with Science. “Now, we have an important hint.”

Researchers first became aware in the 1930s that humans mature more slowly than other apes. A common theory has been that our ancestors experienced a change in the rate at which their children reached maturity because of the nature of their brains.

Slower physical growth would have been more naturally in synch with the construction of a more complex brain, since children who matured physically before their brains could catch up would have been mentally unprepared to survive on their own. On the other hand, an extended childhood would have allowed them to stay in a safe and protected environment for a longer period of time, giving their brains time to grow and develop and adapt to the challenges of an independent adult existence.

To uncover the truth about human evolutionary development in this regard, scientists frequently perform studies on fossilized hominin teeth, which tend to preserve better than bones. Permanent molars in particular are useful, since they produce growth rings just like trees, which can be correlated with the passage of time and the development of the body as a whole.

What such studies have revealed is fascinating. For example, it seems that the teeth of one Australopithecus afarensis toddler, who came from a distant human ancestor that lived 2.4 million years ago in Ethiopia, grew teeth as rapidly as a modern chimpanzee. But by 1.2 million years ago another human ancestor species, Homo antecessor, was producing children whose teeth grew much more slowly (meaning their kids were taking longer to reach adolescence and adulthood).

The Dmanisi youth whose dental remains were examined in this new study has provided fresh and revealing information about what was going on in the period between 2.4 and 1.2 million years ago, suggesting that key developments were taking place over an extended period of time.

Creating a Support Structure for a Longer Childhood

According to the researchers, the Dmanisi child would have depended on adults for food and other types of care for many years after weaning.

Excavations at Dmanisi have shown that the archaic hominin adults who lived there used stone tools, butchered meat, and cared for the elderly, despite having brains that were only a bit larger than those of chimpanzees. This was likely linked to the slowing of childhood development, the researchers suspect, since tool use, meat eating, and the development of a social structure that cared for the vulnerable would have made a longer childhood more viable in the Dminisi society. This wouldn’t have directly caused childhood development to slow down (evolution doesn’t work that way), but it would have enabled such a thing to happen naturally over time.

A group of Homo erectus sharing food with an old and toothless individual who lived several years without teeth, an altruistic behavior associated with the early humans discovered at Dmanisi. (Mauricio Antón / Nature)

But regardless of the reasons, what this new discovery shows is that an increase in brain size and complexity was not the reason why childhood in human ancestors first started getting longer. This may have been a factor later on, but at least 1.77 million years ago the extension of childhood in human ancestors was apparently disconnected from brain development.  

Top image:  X-ray views of the skull and dentition of a young individual of early Homo from Dmanisi, Georgia.           Source: Vincent Beyrand and Paul Tafforeau, European Synchrotron Radiation Facility, Grenoble, France/Nature

By Nathan Falde