We rarely question the things that make Homo sapiens so unique. Our long childhood is an outlier from the other animals on the planet. We are kids longer than any of them. We spent about the same amount of time as bowhead whales. We don't take between 5% and 10% of our lives out to be children, but almost a quarter, because bowhead whales are so large.

Our species has evolved to move the markers of what biologists call "life history" into a different arrangement to other species. We don't live for long, but we are for a long time. The way we build our social and physical worlds is arranged to accommodate this long time of growing up. We can see that our species has chosen to invest in the slow growth of the next generation in ways that no other animal has done.

It has made the way we mate and bond strange as well. Almost all of the animals have been told that monogamy is not feasible. Our monogamous ways have given us genetic co- investors who are important long-term providers. Less competition for mates is something we can see in our bodies as males and females are the same size.

Our genitals are similar to large testes or additional penis length, bones or spikes in size. If we were mouse lemurs, we'd have to testes the size of grapefruits to produce enough sperm to win the game.

If not on the inside, then definitely in the process of coming out, we pour a lot of physical investment into our babies. Evolutionary theory used to hold that the reason human birth is a big risk is because of an inherent conflict between our upright-walking pelvis and a big-brained baby, but we have now studied enough other primate to see that they do things the same way. It's possible that the problem is caused by the amount of resources we invest in our babies in the womb. Human babies are fat and big-brained, which takes a lot of resources. Pre-eclampsia and gestational diabetes can be caused by our reproductive system's unique baby-led pregnancies that can be prioritised over the mother.

Our babies are taken off the breast much sooner than any of our primate relatives. We are not mature until years after other great apes, even ones bigger than us like gorillas, but we are able to nurse our babies for just two to four years, which is much less time than orangutans. We would like to spend more time in the later stages of childhood. We spend our time in the part of childhood where we are surrounded by friends, family and our societies. The most important thing a child can do is learn, and that's why we take our time.

The relationships and skills that allow us to survive have to be invested in not just in physical growth, but also in the relationships and skills that allow us to survive. We need all the time we can get to be a better monkey. Female Chimpanzees learn how to spear from their mothers, while bored Japanese macaques learn how to hot-tub. What about us? We have more to learn.

Boys fishing in the Marovo lagoon, Solomon Islands, Pacific.

The study suggests that the Earliest Pacific seafarers were matrilocals.

We are taking that time to teach our children the skills they need to survive and build them up physically. No other species has been able to create a parenting ally like grandmothers. Females in all animals have reproductive cycles throughout their lives. Do you mean us? Time halfway through women's lives gives us an adult with no dependent offspring of her own to invest in the next generation.

The final question is brought about by this. We can see that we have evolved to stay younger and invest more in our children than any other animal has ever done. Where are we going from here? More learning, more training, more time to be a child is what some lucky children need.

From an Akkadian boy sent to school 4,000 years ago, to the kids getting an internship financial boost from the bank of mum and dad today, parents have found new ways to invest in their children. It's a decision for society how long we allow kids to be kids. I was still doing a degree at 30 years old, despite my grandma's opinion that school until 16 was an extravagance. In Afghanistan, girl children are out at 11 and not so long ago we told the children of miners and farmers that 12 was good enough, now we know they can do just as well with the training that used to be the privilege of the rich.

The real question for this forever-childhood is not so much why we want it, but who we allow to benefit from the remarkable evolution of human investment.

  • Built on Bones: 15,000 Years of Urban Life and Death and Growing Up Human is a bioarchaeologist's book.