Compared with other animals, human babies take much longer to learn to walk. Does this have something to do with our big brains?
—Mahmoud Dhaouadi, via e-mail
John Bock, an anthropologist at California State University, Fullerton, provides a reply:
A horse can walk within an hour after birth. A newborn baboon baby can cling to its mother’s hair while she jumps through the trees. Even among our closest evolutionary relatives—chimpanzees and bonobos—babies are far more agile than their human counterparts. That’s because humans are born with brains that are largely immature, leaving babies with little control over their movements. This uniquely human attribute is the result of a lengthy evolutionary battle between big brains and narrow pelvises.
One of the first traits that differentiated humans from our ancestors was upright gait. There are several hypotheses about the emergence of this trait, but it seems to have offered a way to move more efficiently in open environments such as the savanna. Although our earliest human ancestors were very apelike in terms of their brains, their upright gait had changed their pelvis to look much like our modern one. This reshaped pelvis came with a narrower birth canal, making childbirth more difficult.
Meanwhile the new roaming grounds afforded advantages in acquiring resources and negotiating social relationships to those with flexible, problem-solving behavior. Over time, natural selection increased brain size in these early humans. But at some point, the selection for bigger and bigger brains collided head-on, so to speak, with the narrow pelvis. If babies’ heads got any bigger, they would get stuck in the birth canal and kill both mother and child. Although natural selection worked to maximize what could be done—for instance, babies’ heads compress as they twist their way around the bones in the pelvis—there simply is not enough room for a big, mature brain to pass through.
As it turned out, the evolutionary answer was to let the brain keep growing outside the womb before it matures. So in contrast to other mammals, humans have a good bit of development to do after birth. The result is a relatively undeveloped infant who needs lots of care and can do much less for itself than other newborn primates.
Note: This article was originally printed with the title, "Ask The Brains."
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9 Comments
Add CommentAnother just so story masquerading as science.
Reply | Report Abuse | Link to thisAt least this just so story has a touch of logic to it.
Reply | Report Abuse | Link to thisMight it be that with a few millennia of high-percentage C-section births, pregnancies could, on average, lengthen in time allowing for longer in-womb development, since passage through the birth canal would not be necessary? Of course, there are a host of other issues regarding available space. But, if those become non-factors....?
Can't understand why we rush into print at the first stage of the scientific process. A scientific imagination is indeed important for hypothesizing, but that's not the worthy part, indeed to even get a grant to research a particular hypothesis, it should be necessary to advance a much more rigorous argument than the ones we see published here as science. Science should BEGIN with hypothesis testing in a rigorous environment. This is not science we are reading here this is SPECULATION unsupported by any kind of rigor, and devoid of any attempt at replication or even modeling. This is not science.
Reply | Report Abuse | Link to thisFor example, humans are not the only mamals who undergo a lot of development after birth. So do marsupials. I mean, I could speculate that one of the reasons humans take so long to walk is that walking on two legs is a difficult, though superior, exercise, and that we do it without the benefit of a balancing tail. Secondly, a newborn baby doesn't see people walking around on four limbs so he doesn't even get an example to try this easier form of locomotion, as would a foal for example. What I am doing is quite rational, but it is not science, neither is this article.
There is not as much speculation here as one might think. This material is not un-researched even though the logic alone is compelling. This was a simple response to a question NOT a rigorous dissertation on human infant development. If you have a rebuttal now would be a time put it forth.
Reply | Report Abuse | Link to thisMarsupials may be "born" undeveloped but the sac they live in is still very similar to a womb, a comparison is rather far reaching.
Your second point proves the initial arguments. Babies do not "see" locomotion on two or four legs because they "see" very little at all. Visual development in human infant is at best vague colors and shapes.
An interesting possibility on the long term effects of C-sections, But note that in the past few decades the number of premature births is increasing, could a significant shortening of the gestation be result of that as well?
Reply | Report Abuse | Link to thisJust a thought!
berlou:
Reply | Report Abuse | Link to thisScience begins with observation, then we use our logical minds to come up with hypothesis. (unworthy according to you because it does not get monetary support)
Why do you find publication of a hypothesis not science? Would it not be possible for some other person to read a hypothesis and advance it further?
What you call science is what resulted in the genocide of the poor working classes through asbestos exposure, remember the evidence and the hypothesis were there but the rigorous testing remained unfunded. The same can be said about tobacco and now human induced global warming.
There was a time I took a similar stand, if you can not express something mathematically you don't know what you are talking about. Now I know better.
When it comes to money, remember it will fund only that which will return more money. Money funds research to disprove the harmful effects of smoking. Money funds scientists to discredit human induced global warming. Money funds nuclear weaponry. Money funds biological weaponry. Money no longer funds curing disease, it is more profitable to manage disease for the rest of the persons life, so now money funds disease management.
I prefer the hypothesis that humans walk later because their brains are not fully developed than your walking on two legs is difficult theory.
Look up a dictionary. Read the history of science. Then define science.
Why all the fuss?
Reply | Report Abuse | Link to thisWe know that "ontogeny repeats phylogeny" as the embryo/fetus develops in its mother's womb - from a single cell to multiple cells o worm to fish to frog to ....etc. Can't we say that this evolutionary development continues post delivery too in locomotion? Biped locomotion is much later in evolutionary history. So crawl, go on fours, jerk and hop and then stand and walk! I am sorry that the anthropologist answering the question did not consider this simple aspect.
I agree it is difficult to develop an argument within the confines of the few hundred words allowed for an article, or a post. The article should but does not refer to some or any research which supports the hypothesizing. You can't blame the reader for requiring more. Yes, I am well aware that there are a number of quite respectable and venerable hypotheses surviving in the science today. The Scientific American is itself a venerable journal which has seen the publication of some seminal works which have influenced our very existence on this plantet for ever. It should not be surprising therefore if readers approach this journal with a much higher expectation of academic rigor than one would expect in lesser journals. That being said, I have to admit that on re-reading of the article it seems a much more reasonable piece than my first impression, either that or there has been some cyber-editing going on. On the other hand, I may have conflated two different articles in my mind whilst responding , if so I apologize, but that is the danger of responding to anything on the fly in this new medium.
Reply | Report Abuse | Link to thisI agree, mice and rats among many other animals are born with brains that are as undeveloped as a human infant or more so. Our anthropologist needs to refer to brain to body ratios and to the fact that human babies can cling to their mothers with a reflex just like baboons. A view of "helpless" infants of birds (try looking at panda infants!) would be helpful to inform this person of the need to study zoology before getting a Ph.D. in anthropology.
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