This week I’m summing up where this blog has gotten in its first six months. Today’s post looks at the surprising role of Homo habilis is the origins of speech.
When I began this blog I was not sure how seriously to take Louis Leakey’s old discovery of habilis; now I see the species as probably making the crucial step toward today’s talking species. Of course, “the crucial step” metaphor reflects our perspective. They were a self-sufficient species, not humans manqué, that survived for about 600 thousand years (2.4 million to 1.8 million years ago) and saw perhaps 50 thousand generations. To match them, we will have to press on for another 400 thousand years, 20 thousand more generations. (I’m figuring 12 years to a habilis generation, 20 to ours.) They probably did not talk¸ but I wouldn’t be surprised if they said mama. (See: Hey Daddy-O)
Habilis differed from the various Australopithecus species by having a larger brain, making cutting tools from stone flakes, and by inserting a childhood phase between infant and juvenile stages. (See: Infant Babbling) This change made for one of the less known differences between humans and apes; we have children at a faster pace. Apes commonly reproduce only when a living offspring is about 6 years old; humans do it much more quickly, when offspring are three or so. Since the task of the Darwinian struggle is to outproduce your neighbors in numbers of surviving offspring, this increase in the rate of reproduction would seem to be so advantageous that we are left wondering why apes are so much slower to reproduce.
The answer appears to be in the phrase “surviving offspring.” It takes ape infants a long time to become independent enough to survive without their mother’s regular attention. They are not weaned until they are about six. At that time the permanent teeth begin to appear and they can eat by themselves. We can imagine the teeth erupting sooner and the weaning taking place sooner, but changes in life stages do not come easily to a species because they mean altering the balance in trade offs that mark a species. The fertility rate for any primate reflects a balance between the offspring’s likelihood of survival and the period of infertility when the mother is tending her infant. Earlier weaning enables a mother to reproduce more quickly, but it also means a reduction in the growth time and learning time for the offspring. That reduction probably means a reduced chance at survival.
Natural selection quickly finds the optimal balance point. Lengthening the period before weaning is good for the offspring, but reduces the number of infants a mother can have during her reproductive years. Decreasing the period between live births is in the mother’s reproductive interests, but reduces the chance that an offspring will reach adulthood. Once an optimal balance has been found something dramatic is required to change it.
The introduction of childhood that began with Homo habilis reduced the period between live births, but that change can only have happened if there was some simultaneous reduction in the risk of survival to adulthood.
How was that free lunch achieved? It is a mystery, but this blog hypothesizes that some kind of increased emotional bonding developed between children and the group’s females. That way the burden of tending to the child could be shared by the group rather than imposed individually on each mother. For example, a weaned child that does not yet have its permanent teeth must eat a soft baby food. If each mother must suckle her younger infant while preparing baby food for her still-dependent child, the burden of child rearing increases and seems too great to justify the early weaning. But if the group’s females as a whole are helping prepare baby food, you get an increase in the probability of the mother becoming pregnant again without a commensurate decrease in the probability of the growing child’s survival.
It is hard to understate what a radical change such a basis for survival would entail. In fact, the chief argument against it is that it is so radical. Adoption is such a natural part of human society that many people find it hard to believe it is essentially unknown in most of the animal world. Every February tourists on the Serengeti see the wildebeest calving. Hundreds of thousands of calves are born in a week. Inevitably, some of them become separated from their mothers and the tourists assume some motherless adult will adopt and raise the foundling, but it almost never happens. In the struggle to pass on one’s own genes, the genes of somebody else’s child are none of your concern.
Somewhere between today and the last common ancestor of chimps and humans, we did develop a sympathy for the parentless child and an affection for the neighbor’s child. We help out and, when worst comes to worst, even adopt. I sure did not imagine any such thing before starting this blog, but now I think it likely that the change began long ago with Homo habilis. General bonds of sympathy and affection began by the baby making meaningless cooing and babbling sounds. Other vocalizations that could trace to the same period include laughter and sobbing. The distinctive result of babbling, laughing, and sobbing is a powerful social bonding that pays off very strongly for a youngster by increasing the adult support.
My argument for putting the change at this point in the human story is:
- Locke & Bogin’s work (abstract here) showing that introduction of the childhood stage into human life history began with H. habilis.
- The empirical evidence that babbling precedes speech. Babbling is well known to begin in H. sapiens children before they start speaking words, so babbling may well have evolved first. This likelihood increases when you realize that the distinctive characteristic of human speech—the union of consonant with vowel—is introduced at the babbling stage. It is that sound pattern that makes babbling seem like a precursor to speech rather than some distinctive communication of its own.
- Speech depends on a triplet of speaker, listener, and topic. To create such a trio, speakers and listeners have to develop some interest in one another—a willingness to speak up and a willingness to listen. The emotional attachment encouraged by babbling makes an attetional triplet conceivable.
- H. habilis was a remarkably successful species. Its fossils have been found as far away as Indonesia, indicating that it not only kept up its replacement rate but increased it, so that there was a surplus of individuals able to expand their original territory. This better rate of reproduction is probably enough by itself to explain what happened to the Australopithecines, and must reflect something radically new in the struggle to survive.
Babbling as we know it today is a fleeting stage, replaced very quickly with the use of single words. Presumably, it was more important and prominent in a habilis troop. We can imagine, for example, an experienced maker of stone flake cutting tools teaching a youngster how to make a slicer. The youth chips away, making mistakes. The elder shows the correct technique. It is frustrating, confusing work, but the two stay focused and reasonably calm because they are babbling, making meaningless, reassuring sounds as they work together. It marks a kind of relationship unprecedented in the biological world.