The cerebrum, relative to total body size, which is a defining feature of our species, makes gestation and gestation particularly interesting to paleoanthropologists like me. wise man large skulls contribute to our difficult labor and delivery. But it’s the big brains that really make our species take off.
In particular, my colleagues and I wanted to know how fast our ancestors’ brains grew before birth. Was it similar to fetal brain growth today? Exploring when prenatal growth and pregnancy became human-like could help reveal when and how our ancestors’ brains became more like ours than those of our ape relatives.
To investigate the evolution of prenatal growth rates, we focused on the in-utero development of teeth – which become ossified. By building a mathematical model using the relative length of molars, we were able to track evolutionary changes in prenatal growth rates in the fossil record. Based on our model, it appears that pregnancy and prenatal growth became more human-like than chimpanzee-like nearly 1 million years ago.
Pregnancy and the human brain
Pregnancy and gestation are important periods – they direct future growth and development and determine the biological course of life.
But human pregnancy, and especially labor and delivery, takes a lot of energy and is often dangerous. The large fetal brain needs a lot of nutrients during development. The rate of embryonic growth during pregnancy, also known as prenatal growth rate, takes a metabolic and physiological toll on the gestational parent. And the tight fit of the child’s head and shoulders through the pelvic canal during labor can lead to death, for both mother and child.
Balancing those potential drawbacks, there must be a very good reason for having such big heads. The justification is all the skills that come with having a large human brain. The evolution of our big brain has contributed to the dominance of our species and is associated with increased use of technology and tools, creation of art and the ability to survive in diverse landscapes, among other things.
The timing and sequence of events that led to the evolution of our cerebrums is intertwined with the ability to find and process more resources, for example through the use of tools and cooperative group work.
By examining changes in prenatal growth, we are also examining changes in the way parents gathered and distributed food resources to their offspring. These increasing resources would also have contributed to the evolution of an even larger brain. Understanding when prenatal growth and pregnancy became human simultaneously reveals information about when and how our brains did, too.
Humans have the highest prenatal growth rate of any primate living today, at 0.41 ounces/day (11.58 grams/day). For example, gorillas have a much larger adult body size than humans, but their prenatal growth rate is only 0.29 ounces/day (8.16 grams/day). Since more than a quarter of all human brain growth is completed during pregnancy, the rate of prenatal growth is directly related to how large an adult brain grows. How and when A wise manDeveloped high prenatal growth rate has been a mystery until now.
What teeth can tell about prenatal growth
Researchers have spent centuries studying variation in fossilized skeletal remains. Unfortunately, brains — let alone pregnancy and prenatal growth rate — don’t freeze.
But my colleagues and I started thinking about how teeth develop very, very early in the womb. Your permanent adult teeth started developing long before you were born, when you were just a 20-week-old fetus. Tooth enamel is over 95% inorganic, and the vast majority of everything we see in the vertebrate fossil record is or has teeth.
Building on this realization, we decided to investigate the relationship between prenatal growth rate, brain size and the length of teeth.
We measured the teeth of 608 recently living primates from skeletal collections around the world. We compared those measurements to prenatal growth rates that we calculated from average gestation length and mass at birth for each species. We also looked at endocranial volume – essentially how much space there is in the skull – as a proxy for brain size.
We found that the rate of prenatal growth is significantly correlated with both adult brain size and relative tooth length, in monkeys and apes.
Because prenatal growth is so strongly correlated with relative molar lengths, we were able to use this statistical relationship to generate a mathematical equation that predicts the prenatal growth rate of teeth alone. With this equation, we can take a pair of molar teeth from an extinct fossil species and reconstruct exactly how fast their offspring grew during gestation.
Using our new method, we then reconstructed prenatal growth rates for 13 fossil species, building a timeline of changes over the past 6 million years of human and human evolution. “Hominid” describes all species on the human side of the family tree after the split about 6 million to 8 million years ago from the common ancestor we shared with chimpanzees. From our new research, we now know that the prenatal growth rate increased during hominin evolution, reaching a human rate that exceeds what we see in all other apes less than 1 million years ago.
A fully human prenatal growth rate appeared with the evolution of our species A wise man only about 200,000 years ago. But other hominid species that lived in the past 200,000 years, such as Neanderthals, also had “human” prenatal growth rates. Which genes were involved in these changes in growth rate remains to be investigated.
Comparison means teeth now reveal even more
Even with just a few teeth and part of the jaw, a trained expert can tell countless things about an extinct individual – what species it was, what kind of diet it ate, whether it competed for mates, how old it was when it died, whether it had serious health problems or not and more.
Now we can add to that list for the first time, knowing what gestation and gestation were like for that individual and other members of its species. Teeth may even indicate the rise of human consciousness indirectly, via the changing brain size.
Interestingly, our model suggests that prenatal growth rates began to increase well before the emergence of our A wise man types. We can hypothesize that having a fast prenatal growth rate was necessary to grow that big brain and develop human consciousness and cognitive skills.
These are the types of general questions that we can now formulate in this study – all from just a few teeth.