(Picture from here.)
There have been a lot of media attention on what makes us human. Alan Alda hosted The Human Spark. Nova did a series on it as well. I figure I'm at least as uninformed and ignorant as Fox (or as I like to say Faux) News. I'm going out on a limb today and put my oar in the water.
Much of the attention to where we came from is based on what we are now.
I'm not so sure this is a good way to think about it.
While that is interesting-- not many other primates blog, for example-- it's often putting the cart before the horse. I've seen a lot of shows, articles, books, that push who we are now back to what we were. Certainly, what we were was the enabling animal that became what we are. But we always have to remember in evolution a few things:
- Selection is against individuals, not traits. The traits can enable successful selection but the whole organism has to reproduce.
- What enabled successful selection in the ancestor may or may not be the same thing enabling successful selection in the descendant.
- The ancestor was also a successful product of evolution.
- Anything to be selected against had to be in place prior to the act of selection.
We know Hairy Joe was successful-- we're his descendants after all. But we need to remember that what made him successful isn't likely to be what has made us so successful.
So, what was it about Hairy Joe that was successful (or at least neutral) in its own right at the time that enabled him to evolve into us?
Well, there were a bunch of things: shoulders, hips, thumb, etc. But, because we like our brains let's dwell on that.
One of the characteristics of human development is how bloody long it takes. Human infants are born after a gestation period not too dissimilar to chimps (8 months to 9 months) . A chimp infant learns to hold on to the mother soon after birth. Humans can't even turn over prior to two months of age. We can safely presume that modern humans are born more immature than our closest relative.
Add in to the fact that human brains keep growing long after birth-- much longer than chimps. From one of the above shows I learned that chimp skulls fuse solidly before age five. Humans don't fuse solidly until they're thirty. Humans retain immature characteristics long after birth.
This is called neoteny.
About 6-11% of human births are preterm-- prior to 37 weeks. I looked for statistics for other mammals but didn't find them. But I suspect that the human rate is higher. It makes sense. Unlike most other mammals humans are in a position to accommodate a variability in gestation period. After all, what's the difference between a baby born at 36 weeks and a baby born at 37 weeks? Both are pretty helpless. Apply the same logic to a pig or a cow where the animal be able to walk, follow the mother, etc., from birth. There's no way a cow can look after a helpless infant. Herbivores have to go where the food is and can't bring it back to the cave and give it back.
There's a significant advantage to neoteny-- birds do it all the time. The hatchlings aren't a lot more capable than a new baby. Predator babies (think kittens and puppies) can afford to be born a little helpless. The mother can bring food to the baby-- the same way birds do. If an animal is set up for it, neoteny is great. Smaller infants. More of them.
Not too small and helpless and not for too long. The mother is tied to the den where those puppies are and she can't wait forever. The cost of predation is high and most predators don't have a big support group.
Our long legged ancestors had a capability possessed by neither lion nor lamb: hands. Like chimps, our ancestors could carry an infant with them. They could carry their offspring to the food source. Gorillas do that now. When Hairy Joe went out on the savannah, he was already capable of rearing a less mature offspring than a lot of his neighbors.
Remember what I said about trains that can be selected against having to be in place before selection occurs. Hairy Joe had to have those traits available for selection. This meant a lot of those components we think are peculiarly human had to be present or latent.
There are a lot of things that helped us along: duplication of the genes for salivary amylase, modifications of the shoulder, the shift to upright posture (which also gave us our thumbs), several genes involved in brain development. But I think while these have aided in our evolution since we started on our path I'm not so convinced they started us on our path.
But I think neoteny is key.
Variability in birth timing is something that shows up in chimps as well as humans. We know we needed it to get here since selection for immature infants can't happen without it. The same social paradigms we share with chimps allows for some variability in infant maturity. An increase in this variability could be tolerated if it favors selection-- which it did in our ancestors.
I am not saying we didn't need the other things that have brought us where we are. Variability in brain size, enabling brain size to be selected for, was pretty nice to have. Nor am I saying that preterm birth is always good thing-- it isn't. Our species tolerates a variability in gestation that I don't think other species do. And it's not just because we have better technology. I think it's part of who we were and who we are. It's an evolutionary opportunity like any other pattern of variation.
I'm saying that out in the veldt Hairy Jolene gave birth to a child a little bit earlier than her sister and she cared for it just fine.
It took a little longer to mature than the other kids but when it was all grown up, it happened to be a little bit smarter.