A Powerful Thirst

Water drove human evolution, argues a controversial new book

Paleoanthropologists, and some evolutionary ecologists, study the prehistory of our human past, take the stuff of excavations—ancient skulls, fossil bones, stone tools—and create a narrative, or theory, to fit it. This is a long-standing practice. And now, there is a new theory that is sure to draw both criticism and acclaim. It comes from Clive Finlayson, the co-director of the Gibraltar Caves Project and 2003 member of the Order of the British Empire (for archaeological and museum services in Gibraltar), as presented in his new book, The Improbable Primate: How Water Shaped Human Evolution.

The title leaves little doubt what Finlayson has in store for the reader with this double-barrelled work of popular science. The first barrel, the improbable primate part, refers to Finlayson’s hypothesis—that there is, and only ever was, one Homo species, an improbable one from which we evolved; the second barrel, the water part, relates to Finlayson’s “water optimization” hypothesis—that our evolution was driven by the changing environment and, most critically, by access to water.

Archaeology is a messy business, and I am not just referring to digging in the dirt; conflicting opinions are rampant. Entire academic careers are built on hypotheses that have been passed down by generations of student acolytes. Paleoanthropology is no different; here it is the “lumpers” and the “splitters” that vie for supremacy. “Lumpers” place all hominins into one, or a very few, Homo species; “splitters” prefer to create a new species for significantly varied fossil hominin1. Chris Stringer, an eminent British anthropologist and proponent of the splitter theory, proposed in his “Out-of-Africa” model that modern humans originated in Africa around 200,000 years ago and then, much more recently, dispersed throughout the world replacing all other earlier Homo species including the Neanderthals2. Moreover, in evolutionary biology, an ongoing debate rages between genetic versus environmental determinists—or nature versus nurture. Finlayson blasts into both of these debates, with mixed success. Although his work may be controversial and, at times, in need of more supporting evidence, his theory is a brave one and worthy of consideration.

In the book’s preface, Finlayson loads up his shotgun. It is clear on the very first page that he is on the team of Ernst Mayr, a leading evolutionary biologist of the 20th century and author of Systematics and the Origin of Species, which outlines the biological concept of species and genetic drift. We know Finlayson is on the lumper team when he quotes Mayr: “Never more than one species of man existed on the earth at any one time.” He further quotes Mayr in justification for calling the new genus Homo an “improbable primate”: “the arrival of the fully upright human marked a significant and unprecedented departure from anything that had come before.”

That is Finlayson’s first barrel, that all Homo species—Homo erectus, Homo ergaster, Homo neanderthalensis, Homo heidelbergensis, Denisovans and so on—although highly variable, were all capable of interbreeding and producing viable offspring, and thus are lumped into one species3. We are related to them all, he is saying, with the possible exception of Homo floresiensis. Furthermore, as he intends to prove, our existence is highly improbable.

Throughout The Improbable Primate, Finlayson refers to an area that extends across much of southern Asia, Africa and Australia as “a region of relatively continuous human presence,” and another in Eurasia between 40° N and 50° N, which was colonized from a core in northeast Africa and the Middle East. And although he calls these regions southern and northern “Middle Earth” respectively, this is not a story from Lord of the Rings.

At this point you might be forgiven for thinking this book is simply about semantics; how paleoanthropologists define the concept of species, itself a human-conceived percept. But there is more to this story.

Finlayson’s second barrel, his water optimization hypothesis, is introduced in the preface. Returning to Middle Earth, he identifies its core as the “cauldron of human evolution.” It is here that we discover his key to the explanation of human evolution, the patchy distribution of water across arid and semi-arid landscapes and Homo’s ability to locate it, utilize it and, more recently, develop it.

So there you have it, Finlayson’s theory of human evolution: first, one species from Homo erectus on and, second, an innovative response to aridity by that species. Gone is the former paleontological definition of species, in which species are determined by physical differences in the fossil record, replaced with Finlayson’s definition: “two lineages that produced viable offspring in significant numbers would … be part of the same species.” Gone, also, is “the typological classification of stone tools” for determining the evolution of Homo species and human behaviour—for how could we say “that we are cognitively superior to our parents and grandparents and their own parents who may not have had aeroplanes, fridges, or the Internet?” Gone, as well, are the concepts of anatomical and behavioural modernity “when what we really mean is most recent.”

And that is just the preface.

Finlayson spends the balance of the book hurtling through time, from seven million to 21,000 years ago and later, from Australia to Lake Chad and beyond, justifying his premise that “the thread of human evolution over 1.8 million years ago has therefore been one of adapting to an increasingly arid world while being tied down to the need to drink water regularly.”

In Chapter 1, Finlayson introduces “The Inverted Panda.” The giant panda succeeded in the world of Pliocene meat-eating pandas by becoming a specialist herbivore that eats bamboo. Humans, like the panda, changed their behaviour radically from our ancestors, but unlike the panda, went from largely plant eating to a more general diet that included meat. Hence, Finlayson labels omnivorous humans “inverted pandas,” which left me wondering what he might call human vegetarians. Reverted inverted pandas?

To continue with the dining experience, Finlayson identifies the critical importance of patchily distributed resources because it was under these conditions that our ancestors’ lightweight, gracile bodies behaved optimally. High-quality, patchily distributed foods promoted —
“-problem-solving, information storage and retrieval abilities of good brains,” as well as the tendency to work in groups. The result: a bipedal, meat-eating primate in an open savannah.

By identifying environmental factors as causal in stimulating human emergence, Finlayson firmly propels himself into the nature-versus-nurture debate.

At this point you might ask what about the other primates that exploit patchily distributed resources in groups. Why didn’t they become big-brained too? To which Finlayson would respond, the key is cooperation. He then adds, with understated black humour, that our big-brained ancestors entered into a brain “arms race.” Stimulated by working in groups we became “better interpreters and manipulators of society.” (These statements echo the thesis my colleague Robert Reid and I propounded in our 2010 book, The Climate Connection: Climate Change and Modern Human Evolution, where we named three key factors—crisis, communication and collaboration—as critical in generating innovative human adaptability, about which more later.)

Moving at warp speed, in the chapter entitled “And the World Changed Forever,” Finlayson covers 2.68 million years in 15 pages, with the title forewarning of an impending drama. Here he boldly states that our “remote ancestors of 7–6 million years ago … would have already held the potential for cultural development.”

Why is that a bold statement?

Although Finlayson does not delve into this, geneticists already know that there is very little variation in the DNA at the nucleotide level between a bonobo chimpanzee and a human: 1.2 percent to be precise, which is little more than the average 0.2 percent range of variation within Homo sapiens. Given that very little of the difference between our ancestors of seven or six million years ago and ourselves occurs at the level of our nucleotide DNA, what makes us different?

The most distinct differences between bonobo chimps and us are hair loss, our increased ability to walk on two legs, new hand anatomy, reduction in facial bone growth and the relative expansion of our neocortex. These all mainly involve differences in the distribution in space and time of hormones and their receptors, and not changes at the molecular level. As humans we also possess complex structured language, artistic abilities, empathy and the potential to perceive and analyze problems. From the time the first Homo sapiens appeared around 200,000 years ago, the human brain has not become larger or more complex in its overall anatomy. However, it could have reorganized at the cellular level, with some areas becoming differentiated, perhaps with an increase in the number of neurons or in complexity4.

So what is bold is that Finlayson pushes the potential for neural complexity back six or seven million years, giving the seven-million-year-old central African fossil hominin Sahelanthropus tchadensis the potential to have language, perceive and analyze problems, create art and express empathy. Just as a distantly placed target becomes harder to hit, so too a distantly placed attribute in an ancestor becomes harder to prove (or disprove).

Yet, interestingly, and perhaps a little confusingly, Finlayson does not confer the “first humans” status until the arrival of Homo sapiens erectus, a subspecies that appeared around 1.7 million years ago, some four-plus million years later. Semantics again, you plead?

Not so, Finlayson argues, and this is where his two barrels conjoin. Homo sapiens erectus were the sole survivors after 1.4 million years because of water. We appeared at the same time the world was experiencing increased climate variability, combined with an overall cooling and drying. Open, drier areas along with reduced rainforest resulted in the increasingly patchy distribution of water.

Unfortunately, however, although overall the planet may have been drying and cooling 1.4 million years ago, do we know whether it was like that everywhere across Finlayson’s Middle Earth?

Probably not. Previous research identifies places where the reverse was likely the case. For example, a 122,000-year time-series climate simulation indicates that cooler temperatures 116,000 years ago brought the onset of glaciation5. However, they also brought a significant reversal in precipitation patterns across Africa and the Middle East. While northern Africa and the Middle East experienced decreased precipitation relative to today, central Africa experienced increased precipitation. By 110,000 years ago, although temperatures were still cool, at about 2° C colder than today, precipitation patterns in Africa had flipped again. Central Africa experienced decreased precipitation relative to today, whereas Northern Africa, the Middle East and South Africa experienced increased precipitation.

Thus, although Finlayson recognizes “changes in geographical range of humans,” his broad climate and resource distribution statements need to be tempered with evidence of regional climate variations through time and space. Such conditions would have had an impact on vegetation productivity, water availability and the capacity for various regions to support hominins.

Finally, Finlayson’s innovative ideas are somewhat weakened by the fact that he does not draw a distinction between adaptation and adaptability. Although he intimates the importance of the environment in human evolution, he is quick to retreat to natural selection as the essential factor, and that does not fully explain the developments he is charting in this book.

Adaptations are genetically fixed mutations; adaptability, on the other hand, is what an organism does to physiologically or behaviourally change. Adaptation occurs over the very long term; adaptability happens in a virtual instant. Adaptation, through natural selection, allows organisms to thrive in a specific habitat; unlike adaptability, it does not provide the opportunity for organisms to adjust to sudden environmental change. For example, the structure of the head is an adaptation, and because anatomy is genetically fixed, very little modification is possible. As Finlayson recognizes, not all genetically fixed changes are suitable to the prevailing environment. Survival, or at least success, would depend on an organism’s degree of physiological and behavioural adaptability. Adaptability, in contrast to inflexible adaptation, is the individual organism’s ability to modify or adjust its physiology or behaviour to changing environmental conditions.

For example, in response to defendants of the Aquatic Ape Theory who argue that sweat production is an inefficient adaptation for hominids in a hot environment, Finlayson states, “this is a fallacy that seems to assume that Nature is perfect. It is not … organisms do the best with what they inherit, and compromises are frequent.” But although he recognizes adaptability, he quickly reverts to adaptation when he states “natural selection drove a ratcheted linked response in which preventing overheating and preventing dehydration fed off each other.”

Not all the changes that humans underwent in our evolution were a consequence of natural selection picking and choosing genes. It is true that the amount of hair in humans is heritable, hence adaptational and gene-determined. Long limbs extend the skin area, and in hot climates this creates greater evaporative cooling of perspiration; thus limb length, being heritable, is considered an adaptation. However, much about humans is present due to behavioural and physiological emergences. The best physiological example of this in the context of Finlayson’s work is sweating: humans acclimatize to high temperatures by reducing perspiration rates, thereby saving water.

Marginal geographical areas are where the thrust of evolution occurred, Finlayson wisely argues, because this is where populations are most stressed. Although he does not explain why, previous research does. Climate change and expanding populations can generate stressful conditions. According to Robert Reid in Biological Emergences: Evolution by Natural Experiment, stress stimulates emergences. Emergence refers here to progressive, or emergent, evolution—the sudden appearance of something new, such as when overcrowding causes locust hoppers to become epigenetically stimulated to fly, or when overpopulation stimulates the sudden emergence of carnivorous salamanders that consume their normal vegetarian brethren.

Finlayson recognizes Homo sapiens’ ability to respond to a changing environment, specifically a reduction in the availability of water in a drying climate. Some of those changes were long-term genetic adaptations, others were abrupt physiological or behavioural self-modifications, such as Finlayson’s inverted panda exemplifying changes in our eating behaviour, as well as sweating, and Homo sapiens’ ability to cooperate and work in groups. Adaptability, the capacity to self-modify, allowed early Homo sapiens to survive and thrive in a rapidly changing environment; adaptability will allow humans to survive a rapidly changing future.

The Improbable Primate is worth reading, not because you will necessarily agree with Finlayson’s theory in its entirety, but because he has created an opportunity for critique, discussion and debate. And because he has the courage to create a new generative human origin theory, one worth telling, reading and bandying about.


  1. Hominin refers to the group consisting of modern humans, extinct human species and all our immediate ancestors (including members of the genera Homo, Australopithecus, Paranthropus and  Ardipithecus), whereas hominid refers to the great apes. For a nice clarification, see this explanation from the Australian Museum

  2. Stringer is now rethinking his recent theory of African origin and replacement because of new Neanderthal and Denisovan genetic evidence, as he discussed in a conversation with Edge in 2011

  3. Finlayson does not clearly explain Denisovans in the book. They are considered a new line of human relatives, close relatives of the Neanderthals, linked to indigenous people in Australia and New Guinea through genetic evidence from hominin fossils found in Denisova cave in the Russian Altai Mountains. 

  4. See Susumo Ohno’s Evolution by Gene Duplication (Springer, 1970). 

  5. See pages 141–207 in Renée Hetherington and Robert Reid’s The Climate Connection: Climate Change and Human Evolution (Cambridge University Press, 2010).