We human beings are peculiar in many ways. In terms of physical form, our most unusual features are determined by our bipedal locomotion. This strange habit of getting around on terra firma on two feet was established very early in the ancestry of our hominid family — possibly as much as 7 million years ago. By about 2 million years ago, our ancestors had already acquired the effectively modern body form that committed them and all their descendants to life in the open, far away from the shelter of the forest. With anatomical ramifications from head to toe, this fateful departure from the quadrupedalism of our remote ancestors set the scene for everything else that was to come.
Once bipedalism was established, the focus of innovation among our precursors shifted from the anatomical to the behavioral and the cognitive. From that fateful 2 million-year point on, the human fossil record documents a dramatic increase in hominid brain sizes over time. We may reasonably suppose that this expansion of a metabolically very expensive organ reflected a capacity for increasingly complex behavior.
To understand what was going on during this period of brain expansion, it may best to start by considering the way in which our brains process information today. Unlike any other animal, we modern human beings deconstruct our perceptions of our surroundings into a vocabulary of discrete symbols that we can shuffle around in our minds according to rules, to make new statements not only about the world as it is, but as it might be. This apparently unique ability resonates in every domain of our experience, including our perception of things we cannot discover through our senses. The upshot is that we spend much of our time in worlds that we construct or reconstruct in our heads, rather than in the world that nature directly presents to us.
Of course, many other animals can recognize symbols and respond to them. And, with appropriate technological assistance, our close relatives the chimpanzees can even string symbols together to make simple statements like “take … red … ball … outside.” But, ultimately, this additive algorithm is a limited one, and it certainly does not permit an ape to re-imagine its own world as we do ours.
The modern human’s capacity for symbolic information-processing truly is unique among living creatures. And this, of course, invites us to inquire just when our precursors acquired such exceptional cognitive traits. Do these traits have deep roots in the past, having emerged insensibly under natural selection as hominid brains enlarged? Or did they appear suddenly and all at once? And, if so, when?
It is important to get the right answers here, because they will directly impact our understanding of the kind of creatures we are. If our ability for symbolic thought was driven gradually into existence by natural selection, as evolutionary psychologists suggest, then our minds would be a collection of tools for solving particular problems faced by our ancestors. If, on the other hand, the acquisition of symbolic thought was sudden, there may be something nondeterministic about how we developed, meaning that our capacity to think abstractly and symbolically might be less constrained by the particular circumstances of our evolution. And that, in turn, may have implications for our capacity to make free choices about how to act, rather than being helplessly driven by ancient biological imperatives.
To answer these questions about human development, we can look back at the archaeological record, the archive of past hominid behaviors. We see those behaviors reflected, albeit very indirectly, in the stone tools our precursors left behind. And, upon closer inspection, it turns out that innovation in stone-working technology, while very significant when it occurred, seems to have been very sporadic indeed.
Once the first simple sharp stone flakes had begun to be produced by archaic-bodied ancestors some time over 2.5 million years ago, there was a wait of at least one million years before a new tool type came along: the carefully-shaped “hand axe.” Produced only after modern-bodied hominids had been around for some time, hand axes were revolutionary. And while they certainly suggest cognitive refinement of some kind, they were not necessarily the products of symbolic minds. The same can also be said of the next technological advance for which we have good archaeological evidence: the “prepared-core” tool, whereby a stone nucleus is carefully shaped until a final blow could detach a blank that could be finished as desired. This new development appeared after an even longer period of stasis, and coincided with the tenure of a fairly large-brained hominid we call Homo heidelbergensis. Once more, these tools suggest that our hominid ancestors were extremely smart, skillful, and resourceful but not necessarily in the specific way that we are today. For there is virtually nothing associated with Homo heidelbergensis that suggests this hominid was manipulating information in a symbolic fashion.
Even the large-brained Homo neanderthalensis bequeathed us precious little to suggest that they routinely engaged in symbolic thought. And, perhaps even more remarkably, neither did the very first members of our anatomically distinctive species Homo sapiens. The earliest Homo sapiens fossils, found at Ethiopian sites that are between 200 and 160 thousand years old, give no archaeological indications that the behavior of our earliest ancestors was significantly differently from that of Neanderthals. Only beginning around 100–80 thousand years ago do we begin to find (also in Africa) intimations that early Homo sapiens began to show symbolic behaviors.
These indications include evidence of bodily ornamentation, highly complex multi-stage technologies, overtly symbolic objects (such as geometrical engravings) and, most importantly, evidence of a new and questing spirit of innovation. Previously, change had been only sporadic and rare. Within a few tens of thousands of years, this newly symbolizing species had left Africa and taken over the entire habitable Old World, occupying environments never penetrated by hominids before. In the process, the new species nudged all its other hominid relatives into extinction, perhaps enabled by an unprecedented competitive prowess due to symbolic reasoning. For the first time since the very beginning, a single hominid species was alone in the world — a species that by 40 thousand years ago had begun to leave an incomparable artistic record of its new cognitive status. Although this new cognitive ability is best documented in the extraordinary Ice Age art of France and Spain, there is also remarkable evidence of art in eastern Asia as well — suggesting that the two artistic traditions may have had an earlier common origin in Africa.
The order of events just recounted strongly suggests that our unique cognitive style was acquired both very recently (in evolutionary terms) and in a single, short-term event. Earlier hominids were undoubtedly getting smarter, as their tools and expanding brains suggest. But our ability for symbolic thought appears to be a recent development. And the fact that this innovation was expressed within the tenure of Homo sapiens suggests that it may have been sparked by the action of a behavioral or cultural stimulus in conjunction with an enabling biology that was already present. Most likely, these biological underpinnings had been acquired as a byproduct of the biological reorganization that gave rise to this anatomically distinctive new species.
So what kind of the stimulus might have been involved? The most plausible hypothesis is that it was the spontaneous invention of language — the ultimate symbolic activity. Like thought, language involves creating and recombining symbols according to rules. Moreover, there is evidence that language can be spontaneously acquired among people who are biologically predisposed for it.
Interestingly, evidence of a late and sudden change in how the brain works comes from the apparent recent diminution of this organ. The smart but almost certainly nonlinguistic Neanderthals had brains of almost exactly the same size as those of Ice Age Homo sapiens (around 1500 cubic centimeters). Today our brains are on average some 12.7 percent smaller (around 1330 cubic centimeters). Brain is an energetically costly tissue; and the recent reduction in size suggests that the symbolic information-processing algorithm turned out to be more economical in terms of energy than its ancestral “brute-force” predecessor (a contrast familiar to any microchip engineer). Previously, increasing intelligence had scaled directly with brain size, producing the enlargement trend discernable over the prior 2 million years. Once the new cognitive rules were in place, less brain tissue was needed.
Thus current archaeological evidence powerfully suggests that the unique human cognitive capacity — the basis for our imaginative and inquiring spirit — is something both radically new and recently acquired. This strongly implies, in turn, that our species has not been gradually fine-tuned over the eons to exhibit specific behaviors. Might this suggest that within the limits determined by our social and economic environments, we are indeed individually responsible for the kinds of lives we lead?
- Does the historical record suggest that our behavior is determined by past biological imperatives, or that we are freely responsible agents?
- Are the roots of our unusual modern human cognitive style buried deep in the hominid past?
- Does high intelligence necessarily mean reasoning in the unusual modern human style?
The respondents raised a variety of very good questions, ranging from the nature and uniqueness of symbolic thought itself (which is really a shorthand for something we don’t fully understand, but that we know is qualitatively distinct from all other modes of information processing we can observe), to how we can empirically recognize this cognitive style in the archaeological record. One of the great difficulties here is that it is almost unimaginable that any creature could make the transition from a non-symbolic, non-linguistic cognitive state to the symbolic, linguistic one that is the hallmark of humankind today.
Indeed, the only reason we have for supposing that such a thing could ever occur, is that it so self-evidently did occur. For there is no rational doubt that we are descended from an ancestor that clearly did not process information in the way in which we do. The magnitude of the change makes it tempting to believe that it must have taken place over a long span of time; and yet, the only way in which to rationally interpret the (admittedly indirect) empirical record we have is to conclude that the transition was not only recent, but that it occurred over a remarkably short span of time. In turn, this suggests that it was not driven by gradual natural selection so that the associated behaviors became specifically encoded into our genes). Instead, the new cognitive style was acquired in an abrupt and emergent event, so that we cannot blame our more bizarre behaviors (infidelity seems to be a favorite) on a mythical “environment of evolutionary adaptedness.” My thanks to everyone for their astute comments.