William H. Calvin's A BRIEF HISTORY OF THE MIND (Oxford, 2004)

Upright Posture but Ape-sized Brains

In the woodland between forest and savanna

The dark woods is not where we want to be. Fairy tales play right into this predisposition of ours, seen even in children without experience of the forest or the savanna. We much prefer a few trees, together with a nice view of some water and grass – which is why waterfront property is now so expensive.

Bonobos would not agree with us. They live in equatorial forest, as do chimps, and they likely find wide-open spaces somewhat threatening, just as we do deep dark forests. There are a few places, such as Senegal in the west of Africa and Malawi to the southeast, where chimps live in woodland settings. In a woodland, the trees are interspersed with open areas. That’s also what you find on the fringe of a forest, as a transition between packed trees and grassland. There are, of course, various mixes such as tree savanna and bush savanna, but let me simplify the real estate into forest, woodland, bushland, grassland, and really arid desert where little grows (like the modern Sahara).

Over a period of about 5 million years, our ancestors presumably made the transition from feeling comfortable in the forests to being reasonably comfortable out on the savanna, where Homo erectus was making a living about 1.8 million years ago. You can tell something about diet from where the fossils are found – not from how the fossil site is today (often eroded badlands), but from the nearby bones of other species whose habitat is known to be forest, woodland, or grassland. For most of that 5 million year transition period, the fauna associated with hominid bones are woodland creatures such as pigs, which root around for underground resources that are characteristic of woodlands, not forests or grasslands.

Big brains come along later in the game. What distinguishes the early hominid from the great ape tends, in the eyes of the physical anthropologists who argue about this sort of thing, to be two things: the reduction of the canine teeth and the acquisition of upright posture.

The pelvis becomes more bowl-shaped. Unlike chimps where the legs go straight down from the hip joint, the hominid knees indent. The hole in the bottom of the skull (where the spinal cord descends into the neck) moves forward, to better balance the head atop a now-vertical spinal column. The neck muscles insert into the skull somewhat differently. Some of those upright characteristics are now seen in hominid fossils as early as 6-7 million years back.

That isn’t to say that they had bipedal locomotion as we know it now. By Homo erectus times, they did, but there may have been five million years of transition between the occasional upright locomotion of chimps and an efficient upright stride. Our present bipedal locomotion is in fact more efficient than the ape’s quadrupedal style, but I doubt that the transition was all about “progress” as some transitional stages seem inefficient. (And Darwinian adaptations for efficiency have to operate on immediate opportunities, not long-term prospects.)

What started upright posture, and why is it associated with woodland habitats? In the old museum tableaus, early man stood upright to peer over the tall grass and brush, all the better to spot prey and predators. While formulated for the savanna theory (the notion that we descended from the trees and strode out into the grasslands), it still works pretty well for the new intermediate stage in woodland. Woodlands were surely comforting when venturing out of the forests, as there was often a handy tree to climb, both for escaping a lion and for nesting at night.

Scavenging is often mentioned as part of the transition to the more serious hunting in Homo erectus times. And woodland is a good place to steal some meat, then run off to climb a tree, so as to avoid conflict with late-arriving lions and hyenas. Running fast is useful, and chimps aren’t very good at it when carrying something. They waddle, shifting their weight ponderously, and so they tire easily after a short burst of running.

Another reason for being upright in the woodlands is sunshine. In a shady forest, it is hard to get overheated. But in a woodland, most places get some sunshine for part of the day, thanks to the openings. This means that you cast a shadow, and the size of the shadow at midday says something about how much of a “heat hit” you are getting from direct sunshine. Stand upright in the tropics and a small dark pool is seen near your feet.

Grazing animals cast big shadows but, unlike the bonobo shown here in the unnatural setting of a zoo “savanna” at midday, their brains are adapted to heat stress. If our brains were subjected to the body temperature of an eland at midday, we’d have a seizure. So one way to avoid excess heating is to present a minimal target to the sun. If you stand up straight, your head and shoulders take the hit, and they’re a lot smaller than your back.

Overheating can also be combated by sweating. Evaporative cooling works best with minimal body hair; sweat that evaporates from a hair doesn’t cool the skin very much, for the same reason that the handle of a pan doesn’t transfer very much heat to the hand that holds it. You want the sweat to stay on bare skin, so the heat transfer does some good in cooling the body beneath.

But the loss of body hair, another one of those things that changed sometime between the apes and us, has an important consequence for posture. Transporting infants is generally accomplished in the quadrupedal monkeys and apes by the infant clinging to the mother’s hair, so she can get around on all fours. Thus the transition to profuse sweating likely resulted in a mother having to use an arm to hang onto the infant, and rearrange her travel posture accordingly.

Another consequence of sweating so much is having to stay close to drinking water. Some animals have kidneys that are very efficient at keeping water from being lost in the urine, but we’re profligate, wasting both water and salt and thus constantly having to seek out such resources. It makes me think that our ancestors were often waterhole predators, in competition with the big cats.

So the early hominid habitat was likely a transition zone between forest and grassland, the place where we adapted to heat stress and learned to eat a different diet. Upright posture was likely a byproduct of such factors. What’s surprising to many of us is that the postural rearrangement comes so early, back when the DNA dating suggests we parted company with the ancestral chimps. And millions of years before bigger brains developed.

Teeth tell tales too. Besides upright posture, it is clear that something was also going on with the teeth. You can tell whether it is an ape or a hominid by the size of the canine teeth. Since big primate canines are primarily for fighting, and threats to fight, smaller ones in the hominid line suggest that something was going on that made such aggression less important.

Was it monogamy, like the gibbons? Probably not, because the size difference between males and females changed in the wrong direction. At both ends of our 5 million year long spectrum from upright apes to Homo erectus, one sees males that average about 15 percent larger than females, same as modern humans. In between, however, most upright ape species so far seem to have males that are almost twice as large as females. (Think gorillas.) In the animal world, such sexual dimorphism is usually because the males fight, to exclude one another from access to females, which makes for gorilla-like harems. In that game, bigger is better. With erectus, the size difference becomes minor, more like today.

I’m not sure what to make of this puzzle. One possibility is that the hominid species with oversized males are not actually our ancestors, that there is an undiscovered lineage somewhere, perhaps outside the Rift Valley, which evolved without significant changes in sexual dimorphism.

And there’s another reason to wonder about that, because the teeth also go back and forth in that 5 million year span, becoming much larger and then much reduced. That may have to do with what there is to eat in woodlands. The lack of all-day shade in the open woodlands means that the sun can dry out the soil, in a way that forest floor plants don’t experience. So woodland plants have a lot of underground storage organs for water and building materials. We call them bulbs, tubers, rhizomes, or just “veggies.” Even chimps that live near woodlands have been seen digging them up in the dry season, if just for their water. Were the chimps to eat them more regularly, the variants with larger cheek teeth might fare better.

But what with Homo erectus at the end of the 5 million year transition having smaller cheek teeth, we again have a back-and-forth situation. Maybe it is just adaptation tracking the diet, maybe it is another sign that we’ve missed a more direct hominid lineage to Homo, with the australopithecines and such off on the side branches. Only more data, earned in the hot and dusty badlands, are likely to settle the issue.

Freeing up the hands presumably had some important effects, as Darwin speculated, but brain size sure didn’t change much for the next few million years. So the thought processes of the bipedal apes may have been no fancier than those of the great apes. There might have been chimplike tool use, but it is difficult to find evidence of making stone tools until the very end of this period, at about 2.6 million years ago. An ape-level mentality might have sufficed for life in the woodlands.

The Homo lineage is a spin-off, to use a modern term, of the lineages of the bipedal woodland apes. It occurs about 2.4 million years ago. The bipedal apes keep going, evolving into more heavily built vegetarians, until they die out about a million years ago. It’s hard not to think of them as a woodland version of the gorillas, specializing themselves into an evolutionary dead end. That’s the usual fate of many species and it tends to be aspects of mind – such as having the omnivore’s wide set of food-finding tactics – that can provide the versatility needed to avoid getting trapped.