What Possible Evolutionary/Biological Purpose Does Toddlerhood Serve?
June 24, 2016 11:04 AM   Subscribe

Why did the toddler phase of human life evolve as it did?

As the mother of a young child, I am constantly shocked by the way that the stages of early human life have evolved. Pregnancy? I was way too huge and exhausted to run from predators. Postpartum? Too exhausted from giving birth to run from predators, not to mention now the proud parent of a tiny infant who couldn't even bother to cling to me of his own accord.
And now the toddler phase? He doesn't wanna run from predators, so now I'm running from predators while carrying a stiff-limbed, shrieking, 32-lb little jerk who is trying to bite me. How did we keep the saber-toothed tigers lurking outside our caves from hearing our toddlers inside, wailing over too much gristle on the yak meat?

This isn't a question of how we survived as a species, but specifically why did we evolve this particular phase of life? Do any other mammals have a life stage in which they have total dependence, a total lack of survival skills, logic, or usefulness, and also are stubbornly set on getting into as much danger as possible at a moment's notice?

While this question is jokey and unscientific, I am serious and interested in scientific responses. I loved this Quora answer on the evolutionary/biological purpose of human menstruation (later expanded in this essay), for instance, but I'm looking for more!
posted by aabbbiee to Society & Culture (23 answers total) 13 users marked this as a favorite
Newly fledged raptors go through this phase, sure. So do kittens and puppies and etc.

Evolution isn't a system of perfection; it's a "good enough" system where "good enough" is "keep you alive long enough for you to add your genes to the pool."
posted by rtha at 11:16 AM on June 24, 2016 [10 favorites]

I am not an anthropologist, but my understanding of the general scientific consensus is that humans are born significantly less-developed (in terms of size, gross motor skills, and cognition) than other animals. Being born earlier means the tiny human requires a much higher level of care for a much longer time than other animals, even other primates.

There's a nice article about this in layman's terms here. The traditional explanation is that human gestation length was limited by not letting the fetus's head get too large to be safely delivered. Evidently the more recent explanation is that the true limitation is that being heavily pregnant is hard on the gestating mother's body, and right around 9 months you reach a point where your body literally can't keep up with the caloric requirements of continuing to grow a tiny human on the inside. So out baby comes, much less developed than other animals, and continues to develop more on the outside for another year or so before s/he can walk, run, and communicate.

Hoping another more learned mefite can chime in (paging Chura Chura?).
posted by iminurmefi at 11:17 AM on June 24, 2016 [14 favorites]

Putting on my ex-Paleoanthropologist hat: I was writing a longer answer, but why bother now: the answer is what iminurmefi wrote.
posted by Special Agent Dale Cooper at 11:19 AM on June 24, 2016

This isn't a question of how we survived as a species, but specifically why did we evolve this particular phase of life?

To stray a little from rtha's excellent point, organisms don't evolve to ways of doing things. Things randomly happen, and some organisms survive either because or despite them. Like, there's no reason whatsoever that vertebrates should have evolved a blind spot. We just did and then evolved a brain that fills it in enough that we live on despite the design flaw.
posted by Etrigan at 11:27 AM on June 24, 2016 [17 favorites]

Many animals have vulnerable offspring, primates most notably have highly dependent young.

As an evolutionary biologist I know explained it to me: We evolve imperfectly, randomly and we're not done. We also evolve for thriving/domination of our surroundings. Humans have been pack/tribal/group animals for a long time. Sacrificing childhood utility for adult mental and resource capacity was good math for those groups of people. They did better than primates with stronger young and diminished adult capacity as a group.

Some theory focus on gestational duration or head-size relative to hips. But the ultimate answer with evolution is that the current arrangement works better than past arrangements.

Also tiny kids are really malleable, they develop the skills they need. Toddlers in hunter gatherer tribes accomplish feats of wilderness survival that we would applaud teenagers in the US for doing.
posted by French Fry at 11:34 AM on June 24, 2016 [13 favorites]

The threat of predators has far less impact on evolution than you imagine that it does. Before you even get into the reality that basically zero modern Western behavior has anything to do with evolution. Culture is so much more important than biology when it comes to keeping a newborn alive or what is expected from toddlers.

French Fry's point about sacrificing childhood utility for adult mental capacity is a primary example of that.

Also a shocking amount of evolution has way more to do with what we think is sexy than anything specific about running from tigers.
posted by Sara C. at 12:01 PM on June 24, 2016

You have to remember that we didn't evolve with isolated mamas or even nuclear family groups in isolation (like, say, mama bears and their cubs). We evolved as a society, with the assumption that a mother would always be in the midst of at least several other adults who were not encumbered by pregnancy and/or toddlers, and would be fighting off the predators so the mother would not have to run.

Yes, there were probably many incidents where predators got mothers and/or toddlers who were not so protected, but evolution is about the vast majority.
posted by timepiece at 12:11 PM on June 24, 2016 [2 favorites]

If you think of toddlerhood as the period of time where an offspring is mobile but dumb, and requires parental (or more) care, then I can think of a bunch of animals.

Murres are a kind of seabird whose offspring fledge using just wing coverts, not flight feathers, meaning they can't actually fly (they sort of glide/bump their way down the cliff in a heart-stopping manner). Once they get to the water, they meet up with their dads and head out to sea to finish growing. There isn't much information about this time period but I can imagine that the young murres are dumb, their dads have to teach them to forage (they were previous just brought whole fish) and swim and fly, and keep them out of trouble while letting them stretch their wings (ha! but really, they do need to practice a lot before they can fly).

Other birds have a period of time after the young have left the nest and are still being fed by their parents. Again, not a well studied time because, while it's pretty easy to find breeding adults and nests, it's tricky to follow families that can move a fair distance and have a lot of need to stay camouflaged.

For all these bird species, it's probably the time of highest mortality. There's some mortality to getting to hatching, and then not getting eaten on the nest but that post-fledge time is when young things are particularly vulnerable.

As for the 'how did we manage?' the answer probably has something to do with the fact we live in family groups where cooperative behaviour meant you weren't just responsible for your one offspring and could rely on fathers, aunts, uncles, grandparents, etc., for extra eyes and protection.
posted by hydrobatidae at 12:11 PM on June 24, 2016 [4 favorites]

Also the movie Babies shows the cultural differences that French Fry mentions above.
posted by hydrobatidae at 12:14 PM on June 24, 2016 [1 favorite]

It's also worth noting that toddlers themselves are significantly better at survival than they look. For example, picky eating and the "freaking out because Mommy isn't visible" thing shows up at the same time as the ability to wander into a patch of poisonous berries gets really strong.

What's funny to me is the period where they can't hold up their own heads and the skull hasn't closed up yet. You'd think we'd have some freaking pouches to compensate, but instead they just have the gripping reflex.

But also, with tool-making marathon-running jealous omnivorous relatives, we can afford a relatively long period of fragility and neediness. As someone said above, it's all about getting to the point where the next generation has produced viable offspring.
posted by SMPA at 12:39 PM on June 24, 2016 [2 favorites]

The toddler stage when a human child walks with a curious rolling walk is analogous with the brief staggering steps of a young fawn or a young calf. For a fawn the period is likely to be measured in merely hours. For a human child it could be about four years.

The phase ends when the human child grows long legs and can suddenly keep up with an adult's pace. This is generally a short lived growth spurt because the child is transitioning from too-short-legged-to-keep-up to too-heavy-to-carry. That stage frequently coincides with spontaneous fear of the dark and belief in monsters, a neurological development that helps keep the kid scared to go far from safety especially when it is dark and predators could be nearby.

There is a theory that human babies became altricial compared to other primates because we had an aquatic phase which also resulted in our fur falling out and a subcutaneous fat layer developing. (If you were to shave a gorilla or a chimpanzee they would look dreadfully gaunt because they lack the fat layer than humans have.) Developing this fat layer may have not only had something to do with spending a lot of time splashing around in water and needing insulation but also due to a diet high in marine oils - You know those omega 3 oils that nutritionists are pushing lately. That's the type of oil some of our ancestors probably started eating a lot of around the same time they were bobbing around neck deep in salt water off the coast of Africa.

There would also have been some significant changes in our proportions around then too Compared to other primates we are long legged and can lock our leg joints, something useful when wading. But one of the biggest differences is our absurdly out-sized brains which just happen to require massive amounts of Omega 3 oils to develop. And that big brain means that if we aren't born before our brain has finished developing, we are going to be born at all.

Whatever happened back then and no one is sure, the human ancestors that got bigger brains and more helpless babies were able to survive better than their brothers and sisters that didn't evolve into our species. Maybe there wasn't much to eat except fish, so the fruit eating brothers and sisters starved. Or maybe the drowned because without the fat layer they weren't as buoyant.

It would have been a slow process, not particularly visible to the participants. They would have noticed that some babies could hang on more strongly than some others, and that some mothers had a worse time giving birth than others, and that every now and again someone would drown, but they would still be taking care of their babies as best as they could and scrabbling around looking for things to eat. It just happened that even though babies got more difficult they didn't get too difficult so some of them survived.

Along with the dreadful handicap of helpless babies we developed into social animals. If a chimpanzee mother dies, other chimpanzees don't adopt her offspring. And nobody else helps her carry her baby or her young child. The mother chimpanzee is the only one who nurses that baby and who carries it (although a tolerant relative might let the youngster climb on them it's not the same thing.) Whereas us humans not only developed into having a relatively high paternal investment in our offspring, we also got into nursing our sister's baby, or our daughter's baby, and carried this to such an extreme that in the absence of a sister capable of lactating we will capture other females to nurse for us, not only human ones but bovine or ovine or caprine.

Part of the reason we have such hugely evolved culture and technology is because it keeps our babies alive. Need to put your baby down to go forage for food? Better find somewhere warm to put her because she won't have your body heat to keep her warm enough. That's motivation enough to develop a house (Keeps the coyotes away) and textiles (sheep's fleece is snugger than grass), an understanding of physics (heat rises, baby will be warmer if you put her a couple of feet up from ground level).

There used to be a whole bunch of solemn books about the evolution of man that explained as if it were definite that humans developed into being bipedal because then they could carry a weapon. Yep, we stand upright so we can carry a rock in each hand. But it seems rather more likely to me that when danger appeared any self-respecting chimpanzee would be best off abandoning the rocks and sprinting on all fours. The only thing important enough to carry with you no matter what, whether climbing cliffs, or trees or wading, or swimming or trekking for miles across grasslands is your own baby. You can get more rocks anywhere.

If you are interested in the Aquatic Ape hypothesis, try looking up Elaine Morgan. She was the writer who got me started on reading about human evolution. Carl Sagan's "The Dragons of Eden" might also be interesting if you are interested in tackling books.
posted by Jane the Brown at 1:28 PM on June 24, 2016 [5 favorites]

In a scenario of actual danger, you'd be surprised by how cooperative toddlers can be.
posted by internet fraud detective squad, station number 9 at 1:29 PM on June 24, 2016 [4 favorites]

I mean, if you've ever gotten intensely afraid around a toddler they will notice and never forget it. My son developed a persistent fear of anything mobile and fuzzy because of the mouse infestation in our apartment (and my fear response to it, which was to grab him to keep him from touching the mice when he saw them.) If there were a predator you were really afraid of, they'd pick up on it and respond accordingly by becoming clingy, quiet, etc.

Obviously not always, but they're better at it than you would imagine given their normal terrible behavior.
posted by internet fraud detective squad, station number 9 at 1:33 PM on June 24, 2016 [13 favorites]

You might be interested in reading works by Frans de Waal, a primatologist. He studies bonobos and observes that they have reached the limit of what a single mom can do to raise kids on her own. He suggests that human anatomy indicates that humans have long practiced monogamous pair bonding -- a la the nuclear family -- so that father and mother were both involved in rearing offspring.

Given that we now have about 7 billion humans on the planet and we bellyache daily about how we are on the verge of destroying it, it seems that prioritizing for brain development has been a reasonably successful survival strategy.
posted by Michele in California at 1:36 PM on June 24, 2016 [1 favorite]

Fawns spend quite a number of weeks in "toddlerhood." Part of the reason for this question is that we have a fawn in our woods that was born this spring. It could walk soon after birth, but it spends its days/nights bedded down in various piles of brush, under cover, away from its foraging mother. The mother returns at regular intervals to nurse and move the fawn to a new hidden spot, and this goes on for many weeks. The fawn is scentless, motionless, and undetectable, so it is safer hidden than scrambling along making a racket next to its mother. These days the fawn is pretty good at running, so if I come upon it, it will explode out of its hiding place and go find mama or go to a predetermined safety zone (I'm not sure which, but it always runs the same direction), but clearly it still can't quite keep up with her. So this question comes from the striking similarities & dissimilarities of my kiddo and the fawn, both of whom are at a similar developmental stage. Both dependent, both lack survival skills, yet my kiddo thinks he's hiding if his eyes are covered with his hands.

The points about group evolution are well taken. I appreciate the thoughtful responses- please keep them coming!
posted by aabbbiee at 2:04 PM on June 24, 2016 [1 favorite]

I just flagged this wonderfully written comment in the ask about how to not let grandpa take kid on boat as I think you intended it for elsewhere:

There's a diversity of opinion about this, much of which probably overlaps. There's the idea, very generally, that the human neocortex has become vastly more powerful in a relatively short number of generations because the survival payoff of our logic, planning, anticipation, communication, social organization etc. significantly outweighs its tremendous cost. This apparently prolonged period of parental dependency (which itself can't be separated from our supposition that humans have been socially organized, not solitary, since well before we were humans) likely relates in some way to the experiential training the neocortex requires to reach this survival payoff plateau. The idea, then, becomes: a really fecund species that self organizes into social cohorts capable of complicated, high-payoff acquisition of food that can get even a few percent of offspring to survive to maturity will be more likely to reproduce and rear a subsequent generation of really fecund high-payoff food finders, and so on. Here's a sample of this kind of thinking, just trace the citations outward. The depressing other side to this is that human women can get pregnant anytime they aren't nursing (again speaking really generally), which means that the need to rebound quickly after the death of one hard-to-keep-alive child is baked in to the system.
posted by late afternoon dreaming hotel at 2:19 PM on June 24, 2016

Generally baby animals serve as food for predator species, as well as sometimes their own species (especially when born sickly or malformed). Why should ours be any different?

Maybe loud anarchy is more of a deterrent to predators than you think? It sure is for me.
posted by Mchelly at 2:50 PM on June 24, 2016

I know this is one of the theories why humans developed menopause and only in females. Basically mom helps her daughters raise their young. I haven't been in the discipline in years so I don't know how popular it is anymore but females in many species share child watching / raising with other female relatives. Particularly non-herbivores who need to hunt. Cats are a good example: one queen will nurse and watch the kittens while the other goes to feed. After all a mother who cannot provide for her children and herself isn't going to be well represented in future generations.
posted by fshgrl at 2:52 PM on June 24, 2016 [3 favorites]

Children between about 2 and 5 are exceptionally good at making their needs known to those around them. It's also around the time that a new baby might join their family, so they need to be annoying enough to get their share of attention, food, and protection, but not so much that they'll get killed and eaten by other adults in their group. So they test the boundaries and keep jusssst to the point where they are safe.
posted by tchemgrrl at 4:59 PM on June 24, 2016 [2 favorites]

One reason for toddlers is training time, trust training, social training and outright education. We don't have enough time for everyone to start from scratch, they have to learn how to operate their clumsy bodies, and make organized sense of the world they are in. They need a good data base to build on, lots of mirroring and reinforcement. Fight or flight mechanism inhibits learning.
posted by Oyéah at 7:56 PM on June 24, 2016

He doesn't wanna run from predators, so now I'm running from predators while carrying a stiff-limbed, shrieking, 32-lb little jerk who is trying to bite me.

And now you see how millions upon millions of toddlers have survived for the past million years...

Evolution didn't protect them from predators. Moms did. (And dads too, no doubt...and cousins, grandmas, other people - you know we used to live in larger groups and stuff...)
posted by Toddles at 8:03 PM on June 24, 2016 [2 favorites]

Do any other mammals have a life stage in which they have total dependence, a total lack of survival skills, logic, or usefulness, and also are stubbornly set on getting into as much danger as possible at a moment's notice?

You might find documentaries about other social mammals entertaining. I've enjoyed both Meerkat Manor and Lions: Spy in the Den. They both feature keep-your-hands-full toddlers. Baby brown and grizzly bears do lots of exploring, too, even though the adults aren't social in the same way.

They're all animals which eat a complex array of foods and are kept relatively safe by adults. The more exploring that they do while you the parent are around, the more that they can learn from your reactions. Yummy? Scary? Interesting? Can you show me how to open this yummy/scary/interesting thing?

But the great apes, other than humans, not so much. Sarah Blaffer Hrdy is a proponent of the "grandmother hypothesis" that fshgrl refers to, and she makes the argument that human toddlers explore in a way that chimp and gorilla toddlers don't because human toddlers are much less likely to be killed by other members of the community. Read her great book Mothers and Others if you're interested in more:
Mother apes in the wild are obsessively anxious about their babies. Frankly, if I were a mother chimpanzee, I would be too. Primatewide, mothers have to worry about strange males, and in both chimpanzees and gorillas, infanticide by males is a major source of infant mortality. But because females typically leave their natal kin to breed in other communities, they have to worry about unrelated and potentially infanticidal females as well. This is especially true of highly omnivorous common chimpanzees, who eat baby gazelles and colobus monkeys when they can get them. Baby chimpanzees are a no less delectable source of proteins and lipids.
Be warned that it gets gory if you keep reading.

The grandmother hypothesis is an alternative to the nuclear family hypothesis that Michele in California mentions. They both agree that human mothers need more help to raise children than chimps do, but they disagree about who was the first to stick around and help the mother out. Either way, your toddler explores more than a chimp toddler does because a) exploring is learning and b) other human adults are more likely to help it than to kill it.

I'm going to go a step further and offer a half-baked theory that isn't peer reviewed: Your toddler is also testing to find out how much social support she/he has, how high up the social class ladder you are. Is there harsh punishment for stepping out of line, for exploring too freely? You're probably in a precarious social position, and it's important for the toddler's survival to learn that. Can the toddler get away with anything, are they coddled by all the adults in the community? You're probably in a great social position, and a toddler who knows it will be able to take full advantage of the reproductive possibilities years down the line.

Lots of other great answers here, and a great question!
posted by clawsoon at 8:45 PM on June 24, 2016 [3 favorites]

Human culture is impressively flexible. I have read some things that examine the differences between African culture and "western"/white culture. There are substantial differences in how they frame expectations concerning family and sexual relationships. So the nuclear family hypothesis and grandmother hypothesis are not necessarily competitors. It is entirely possible they are complementary.

Some scholars suggest that African American culture is as resilient as it is in the face of the shitty legacy of slavery because the cultural expectation they brought with them from Africa that, like a lion pride, the backbone of the family is the women who are blood relatives -- the sisters, mothers, grandmothers and aunts are the people that identify and define the family. In the face of high unemployment rates and high incarceration rates among black males, this matrilineal social organization is both abhorrent to whites, who frame it in terms of immoral behavior, and a source of stability that baffles the white culture that has long sought to crush them at every turn.

Other sources generally note that family is a tremendous source of strength for humans and is amazingly flexible and adaptable to a degree not found in other human organization schemes. Corporations, military groups, not for profits -- none of them is anywhere near as flexible, thus none of them is anywhere near as cohesive, as the human family.

My son, who has never wanted children of his own, but did spend some years wanting me to give him a second sibling, has read up on research into evidence that many creatures will give more support and defense to individuals who share a great deal of their DNA as potentially an alternate means to preserve their DNA. Ants or bees can have many sisters, some of whom share like 50% of their DNA and some of whom share like 75%. The more DNA they share, the more fiercely loyal they are to each other. These sisters do not reproduce. Only the queen reproduces.

As for the actual question of toddlers, toddlers are learning to play. It is well established that the more intelligent an animal (species) is, the more play time is required. Play is a means to intellectual development. Nobility has long engagedvin the pursuit of games like chess, which did not merely idle away the hours not spent tending the field but also provided instruction in Tactics and strategy for political and military maneuverings.

Einstein himself insisted that imagination is more important than knowledge. Toddlers are learning to use their imagination. With the development of imagination, humans literally dream the future into being. We think it and we make it real, profoundly shaping the world around us.
posted by Michele in California at 9:48 AM on June 25, 2016 [1 favorite]

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