Humans take quite a while to grow up. Part of this is biological, as it takes an extra few years for our big brain to mature. Our fancy behaviour is also impacting this. We need to spend an extra few years learning how to be a human. As a result of these factors, humans develop "adult" brains at around 12 years old. This is nearly 5 years later than our close (but small brained) relatives, chimpanzees.
All of this makes tracking growth in ancient human species key. When these species began to grow up slowly could provide a key insight into when they were becoming more "human". For the most part, this research follows the pattern you might expect. Ancient humans with larger brains took longer to grow up than smaller-brained species.
However, there is a notable exception to this pattern. Neanderthals, seem to have grown much faster than us; despite having a brain bigger than ours. Sounds strange? You're not the only one who thought so. Researchers have been studying these wacky Neanderthals to figure out if they really did grow up weirdly.
Neanderthals speeding up
Annoyingly, fossils don't come with labels. So one of the most important jobs of a palaeoanthropologist is figuring out what exactly they've found. Teeth are great for this. They're species unique, which useful for figuring out what you've found.
Also, as you've probably experienced, they change during childhood. Milk teeth are replaced by adult teeth at a fairly consistent rate. So you can also figure out how old they were too. Yes, all of that pain wobbly teeth caused you is scientifically useful!
We've been studying young Neanderthals for a long time. In fact, the first ever Neanderthal fossil found belonged to a child (although it wasn't recognised as a different species for decades). Using teeth to figure out the age of these fossils has revealed something very interesting: Neanderthals appear to have grown up quickly. Fairly mature fossils have been found that still had young teeth.
Remember the link between the rate humans grow up, our brains, and learning? Many think that since the Neanderthals grew up quickly this may have been compromised. Perhaps they didn't spend as long learning to survive as humans. This could explain why we flourished whilst they went extinct. Or, just maybe, these estimates of Neanderthal growth are wrong.
Neanderthals grow up weird
It turns out that not all of Neanderthal growth is different. We've found some Neanderthal babies. They show that at birth, Neanderthals had a brain about as big as human baby (i.e., just larger than a can of coke). However, by the time they'd finished growing up Neanderthals would have had a brain bigger than ours.
There are two obvious ways to achieve this massive brain. Neanderthals either grew for longer or grew faster in the same length of time. Fossils of young Neanderthals (as indicated by their teeth) indicate that they went for the latter. By the age of 1, a Neanderthal brain would be ~50 ccs bigger than a human, and the gap just kept growing. This allowed them to finish with the bigger brains of Neanderthal adults.
Their teeth seem to indicate that it wasn't just their brain growing quickly. Many young Neanderthals have more teeth than they should, with some appearing more than 2 years earlier than in humans. So by the time the brain was getting close to finished so might the Neanderthal childhood.
Compare this to humans. Our brains reach 95% of adult size by age 7. But as you might know, 7-year-olds aren't adult yet. They still have plenty of childhood left. But this might not be the case for the Neanderthals. By that age they could be approaching maturity. And with it, they're also possibly missing out on a whole heap of childhood learning.
Did they really grow up like this?
Right, have you been following what I wrote? Well done, but you might need to forget about it. See, a 7-year-old Neanderthal was discovered at El Sidron in Spain. Well, the Neanderthal was actually 49,000 years old; but he was 7 when he died. And also a he, lovingly named "J1" by the scientists.
Now, remember how 7 is a key point in human development. By then humans have ~95% of their adult brain. Based on the existing model, J1 should also have reached 95% of adult brain size. And perhaps be a lot more developed to boot. Except he wasn't. J1s growth fell within the expected pattern of a modern human 7-year-old.
Crucially, this means his brain - whilst it was the size expected of a human - hadn't reached the same 95% threshold for Neanderthals. Instead, his brain was closer to 87% of the adult Neanderthal size. This would suggest he was growing up at the same rate as modern humans, likely for longer to get to the full Neanderthal brain size at this slow rate. His childhood may have been as long, if not longer, than our own.
All of these strange implications came from the fact they used additional factors to age J1. In particular, they examined his bone growth to determine his age. Repeating this method on some other young Neanderthals indicates they also may have been growing at human-like rates after all.
However, their teeth may have just been developing at slightly different rates, creating the appearance of rapid maturity I spoke about earlier. Whats more, Neanderthal teeth may have been growing at different rates in different populations. Knowing J1 had human-like teeth at 7-years-old doesn't mean they all did.
So whilst Neanderthals and humans did grow up slightly differently, we're a lot more similar than we thought after all.
References
de León, M.S.P., Golovanova, L., Doronichev, V., Romanova, G., Akazawa, T., Kondo, O., Ishida, H. and Zollikofer, C.P., 2008. Neanderthal brain size at birth provides insights into the evolution of human life history. Proceedings of the National Academy of Sciences, 105(37), pp.13764-13768.
Gunz, P., Neubauer, S., Golovanova, L., Doronichev, V., Maureille, B. and Hublin, J.J., 2012. A uniquely modern human pattern of endocranial development. Insights from a new cranial reconstruction of the Neandertal newborn from Mezmaiskaya. Journal of human evolution, 62(2), pp.300-313.
Rosas, A., Ríos, L., Estalrrich, A., Liversidge, H., García-Tabernero, A., Huguet, R., Cardoso, H., Bastir, M., Lalueza-Fox, C., de la Rasilla, M. and Dean, C., 2017. The growth pattern of Neandertals, reconstructed from a juvenile skeleton from El Sidrón (Spain). Science, 357(6357), pp.1282-1287.
Smith, T.M., Tafforeau, P., Reid, D.J., Pouech, J., Lazzari, V., Zermeno, J.P., Guatelli-Steinberg, D., Olejniczak, A.J., Hoffman, A., Radovčić, J. and Makaremi, M., 2010. Dental evidence for ontogenetic differences between modern humans and Neanderthals. Proceedings of the National Academy of Sciences, 107(49), pp.20923-20928.
Zollikofer, C.P.E. and León, M.S.P., 2013. Pandora's growing box: inferring the evolution and development of hominin brains from endocasts. Evolutionary Anthropology: Issues, News, and Reviews, 22(1), pp.20-33.
