The Real Planet of the Apes: A New Story of Human Origins

APES

INTRODUCTION

For more than 150 years now it has been widely accepted among biologists and most other scientists that humans evolved from an ape. Not one that lives today, such as the chimpanzee, our closest living relative, but one that lived millions of years ago. Let me make that perfectly clear. We did not evolve from a chimpanzee, nor did chimps evolve from us. Rather, chimps and humans evolved from an unknown ape that lived before humans and chimpanzees branched off from each other, at least 7 million years ago, to pursue their own evolutionary destinies. What was this ape like? And from what did it evolve? And further back in time, what was the common ancestor of all apes and humans like? And what might studying ancient apes tell us about what makes us human?

These are the questions that have been driving my research since I was a graduate student in the 1980s and that motivate this book. We have known, ever since the work of Charles Darwin and other researchers of the late nineteenth century, that we have a special evolutionary relationship with apes. In a real sense, we are bipedal apes. Darwin and his principal defender, Thomas Henry Huxley, concluded in the 1870s that humans not only share a common ancestor with apes, we share a common ancestor with African apes to the exclusion of all other primates. To put this another way, Darwin and Huxley concluded that chimpanzees and gorillas are more closely related to humans than to orangutans. Orangutans are Asian great apes, a branch of the great apes that split from the common ancestor of the African apes and humans.

Among Darwin’s contemporaries and researchers well into the twentieth century, the idea that African apes could be more closely related to humans than they are to orangs was hard for most scientists to swallow. Many rejected this conclusion, favoring the hypothesis that humans branched off first, and then what we call the great apes—chimps, gorillas, bonobos, and orangs—went off in their own different directions. After all, humans are very different from the apes, physically and mentally. Great apes all look more or less the same, at least superficially. They are large, hairy beasts with long arms, short legs, big jaws and teeth, and small brains compared with humans. Still, all of these early researchers recognized the reality of human evolution and our connection with apes. As we will see, there are in fact many differences among the great apes, and as a group they are not as different from humans as they may first appear.

Today, scientists group modern humans with the great apes. Although we have bigger brains and walk upright on our hind limbs, we share an enormous amount with the apes, from an almost indistinguishable genome to more similarities in our structure and behavior than we share with any other living organism. This is the reason that scientists group us with apes, to the exclusion of all other organisms, in the superfamily Hominoidea. We are especially closely related to the great apes, and so we share with them a place within the family Hominidae. (Figure 0.1 illustrates the relationships among the apes discussed in this book.) We will learn more about how all primates, apes, and humans are related later in this chapter.

In this book, I want to tell the story of ape evolution over approximately 30 million years. In many ways, my account differs from the usual textbook account, but I think it better explains how we got to be the way we are. Indeed, it is impossible to understand and explain the course of human evolution without understanding ape evolution first. Modern human anatomy can only be understood as a direct consequence of having evolved from an ape. We did not evolve from any living ape, but the anatomy of the common ancestor we share with apes sets the stage for human evolution. Everything from the structure of our teeth to our brains, our dexterous hands, our upright posture, and even our reproductive biology all have precursors in the anatomy of our ape ancestor. Many of these attributes are still found in living great apes, which is one of the reasons we study them so thoroughly.

FIGURE 0.1. Evolutionary history of the hominoids. There are almost no direct ancestors in this diagram because it is very difficult to say if a fossil represents a direct ancestor or a side branch. LCA, last common ancestor; 1, hominoid (all apes); 2, hominid (great apes and humans); 3, hominine (African apes and humans). The fossil ape Oreopithecus and Hylobates (gibbons and siamangs) cannot be linked confidently to any known branch of the hominoid evolutionary bush. Nsungwepithecus may be the oldest known cercopithecoid (Old World monkey).

When I began my study of fossil apes from Europe, nearly all of them were attributed to the genus Dryopithecus, an animal scientists considered to be far removed from the central court of great ape and human evolution—a side branch in early ape evolution. I found no reason to challenge this conclusion at the time. However, I did come to the conclusion as I was completing my thesis that these fossil apes are great apes. In other words, they are hominids (the group that includes orangs, gorillas, chimps, bonobos, and humans.) With the discovery of more fossils, especially from Spain but also from Hungary, the view that European apes are more central to the question of hominid origins became more widely accepted. Before this, most scientists believed that European apes were just an interesting side story. Fossil apes were better known at the time in Africa, and as I said earlier, the overwhelming consensus was that great ape and human evolution was a mainly African story.

As I continued to work on European apes I had another idea. These fossil apes are not only great apes but also African apes (hominines.) This was a new idea—and it is not as widely accepted today as the conclusion that European apes are great apes. The reason this new idea is controversial is the same old story—every event of any significance in the evolutionary history of apes and humans was widely considered to have occurred in Africa. It took many new discoveries and detailed research on these new specimens to build the case for the presence of African apes in Europe.

Nevertheless, I began to wonder what African apes were doing in Europe 9 to 12.5 million years ago. At first I thought that they had dispersed into Europe from Africa, a sort of excursion by a side branch of African apes that eventually led to extinction. Finally it occurred to me that the ancestor of the African apes and humans may actually have evolved in Europe instead of Africa. While it was a radical departure from widely accepted reconstructions of ape and human evolutionary history, I was intrigued by the African ape features I found in European fossil apes and the complete lack of evidence for fossil great apes in Africa during the same time period. I found evidence that European Miocene apes were more advanced (or derived, in scientific terminology) than apes from the early Miocene of Africa and share characters with living African apes and humans. I hypothesized that African apes evolved in Europe and moved to Africa, not the other way around. Not to overdramatize the point, but I do think of this as a eureka moment that has to some extent defined the trajectory of my career afterward.

I am determined to falsify this hypothesis. That may sound strange. But we cannot really prove anything in paleontology. We can only try to find evidence inconsistent with prevailing theories. The only way to formally test my hypothesis is to seek to disprove it. To do so, I have been testing this hypothesis with new specimens. Many European fossil apes share characteristics of orangutans, which I interpret as primitive (they evolved first). But they all have features of African apes, as we shall see later. As surprising as it is, there is strong evidence, which I will reveal, to support my hypothesis. This is, to be honest, a hypothesis that goes against most opinion all the way back to Darwin, though as we shall see, Darwin was more open-minded than many persons are today (He was receptive to the idea that Dryopithecus might have a connection to African apes). To summarize the major conclusions of this book, my research and that of many colleagues has led me over the last thirty years to a number of conclusions. Apes evolved in Africa from ancestors of African origin (e.g., Aegyptopithecus). By about 20 million years ago, primitive apes, more monkey-like than apelike, began to flourish in Africa. Among these apes, one was better equipped to disperse to more seasonal climates (Eurasia). This ape, with its abilities to exploit a broader range of resources than the first apes, was poised to take on Eurasia. The ecological conditions in Eurasia selected for new adaptations in apes. The ape that dispersed into Eurasia began to evolve novel features related to diet and positional behavior, eventually splitting into the two major groups of living great apes, the pongines in Asia and the hominines in Europe. At the same time, the fossil trail in Africa went cold temporarily (there are no fossils), while in Eurasia ecological conditions favored further changes in locomotion (suspension) and increases in brain size. Large-brained and suspensory apes flourish until a progressively cooling and drying Eurasia eventually caught up with them. Many went extinct but a few were able to disperse south, tracking the forests retreating toward the equator. The ancestors of orangutans ended up in Southeast Asia while the common ancestor of the African apes and humans settled somewhere in the African tropics. By roughly 10 million years ago, gorillas separated from the common ancestor of chimps and humans, and by about 7 million years ago, chimps and humans diverged. It is at that point that the human lineage emerged. If you find the narrative confusing as you read the book, don’t give up. Come back here and remind yourself of the major events in ape and early human evolution. It is a story, and I hope it will make sense to all of you. Before we embark on this grand narrative, it is important to learn more about primates and apes, and crucially what characteristics we humans share with them.

HUMANS ARE PRIMATES

We belong to the zoological order known as the Primates. The classification of primates can be complicated, so I will make it simple here. Primates are traditionally divided into prosimians and anthropoids. Prosimians include lemurs, lorises, and galagos or bush babies, among others, and tarsiers.¹ Anthropoids include New World monkeys, Old World monkeys, apes, and humans. Nearly all anthropoids are daytime active and most are larger than prosimians. They have larger brains and rely less on insects and more on fruit and leaves. Most anthropoids are highly social and even more visually oriented than prosimians. Most anthropoids are also tree dwelling, but some spend time on the ground, especially the larger Old World monkeys (baboons) and African apes.

All primates are intelligent, dexterous, clever, vision-oriented animals, mostly tree dwelling, with grasping hands and feet (except humans; our feet have lost this ability). Brain size and eye-hand coordination is generally greater than in other mammals of similar size. It is clear that the evolution of the primates forms the foundation for the evolution of the apes and humans.

Most people use the word monkey to refer to those hairy, four-legged critters that kind of look like us and can be trained to do clever things. It is common to refer to chimps or gorillas as monkeys, but in fact, apes and monkeys are very different. First of all, there are two groups of animals that biologists call monkeys. One of them, the New World monkeys, live, as the name suggests, in the New World (Mexico and Central and South America but not the United States or Canada). The other kind of monkey lives in Africa and Asia, and we call them the Old World monkeys. While we refer to both groups colloquially as monkeys, New World monkeys are distinct and evolved separately from the Old World monkeys, apes, and humans. So, from now on, when I refer to monkeys, I am talking about Old World monkeys (see plate 1).

Old World monkeys, apes, and humans all fall within the zoological category of the catarrhines. New World monkeys are in a different group, having diverged before Old World monkeys and apes split. Roughly 35 million years ago, there was a population of primates that gave rise to the catarrhines to the exclusion of all other primates. In other words, we and the other catarrhines branched off from the common ancestor of the New World monkeys at that time and have since evolved in our own way. Since that initial branching event, Old World monkeys and apes have branched off from one another, as have each of the lineages of living apes and humans in turn.

Old World monkeys include baboons and macaques, which are the most common, but many other species exist. Monkeys are intelligent and very flexible in their behavior. These traits allow them to adapt to life in harsh climates, such as the snowy mountains of Japan, though most species, like the apes, live in the topics or subtropics. They generally live in social groups with complex hierarchies in which rank is important and can be inherited. The offspring of a high-ranking monkey are likely to be high ranking, too. Monkeys almost always give birth to one infant per pregnancy and invest a great deal of time raising and nurturing their young.

Compared with other mammals of similar size, monkeys generally have larger brains. They are very agile, and while some are more adapted to an arboreal (tree-living) lifestyle and others are more terrestrial (ground-dwelling), all monkeys move quickly and adeptly both on the ground and in the trees. Stay clear of them if you go on a safari or visit a place where they run free; their antics may be cute, but many have lost their fear of humans and they will not hesitate to bite. In fact, in the West we tend to think of monkeys as adorable fuzzy creatures, but where they live side by side with humans, they are often not very well liked, mainly because they steal food, destroy property, and raid crops.

So monkeys are intelligent and adaptable, traits that have allowed them to thrive alongside humans. It is no exaggeration to say that monkeys are among the most intelligent creatures on the planet. However, compared with monkeys, apes are in another category altogether.

Most of the apes are much larger in both body size and brain size than monkeys and all apes lack tails. (I am talking about living or extant apes here; some of these attributes were not present in the earliest apes, as we shall see.) Their arms are long (longer than their legs), allowing them to swing below the branches, whereas monkeys mostly walk along the tops of the branches. Female apes go through a menstrual cycle rather than a more seasonal reproductive cycle (estrous), as female monkeys do. Apes live longer and have larger, more complex societies and more complex and social brains. Apes score higher than any monkey in lab tests of intelligence, and some researchers have even claimed to have been able to teach apes to communicate in a rudimentary language. Although there are detractors regarding the ape language experiments, it is clear that apes are capable of much more cognitively challenging tasks than monkeys. In the wild, great apes, especially chimpanzees, make and use tools in foraging. All great apes build nests to be comfortable and safe at night in the trees or on the ground, they make umbrellas and other devices to protect themselves from the elements, and they devise novel and intelligent solutions to the problems they face. In my opinion, ape intelligence, specifically great ape intelligence, is an order of magnitude above that of any monkey and makes an obvious comparison with early humans. That is really the main reason we study great ape behavior in the wild. They surely represent something close to the way the earliest humans, which were great apes themselves, behaved. We did not evolve from a living great ape, but the earliest human species anatomically resembled living great apes and surely behaved much more like living great apes than living humans. Why we have changed so much and apes so little is possibly the biggest puzzle in paleoanthropology.

Scientists divide the living species of apes into two groups. Lesser apes include the gibbons and a less familiar primate called the siamang. Gibbons and siamangs are placed in the family Hylobatidae, or hylobatids, more informally. As we learned earlier, the great apes include chimpanzees, bonobos (formerly known as pygmy chimps), gorillas, orangutans, and humans. They are collectively known as the Hominidae, or hominids. The vast majority of paleoanthropologists recognize this dichotomy and are okay placing humans and great apes in the same family (hominids), though there are some who continue to elevate humans to a unique family (hominid) and place great apes in another group called the pongids. This is an evolutionarily artificial classification that places the unique adaptations of humans above the genetic and anatomical evidence of our relationships with the great apes. It runs counter to normal taxonomic practice and, I would say, plays into the hands of those who refuse to accept that humans evolved from an apelike species, or evolved at all. Here I use hominids to refer to the great apes and humans (see plate 2).

The recognition of the similarities between great apes and humans is remarkable and recent. Up until the 1990s most researchers reserved the word hominid for humans and our ancestors—all species on our branch of the family tree after we split off from the last common ancestor we share with a great ape. In most of my publications through the mid-1990s, I had to justify the use of the term hominid as employed here as if it was controversial and confusing. Today, reserving the term hominid for humans has become controversial and confusing. Most researchers, myself included, place African apes (chimpanzees, bonobos, and gorillas) and humans (we and our fossil relatives) in the same subfamily, the Homininae (hominines) (table 0.1). A few researchers even advocate including the genus of chimpanzees and bonobos (Pan) within our genus Homo, as a subgenus. Most researchers today agree that chimps and gorillas are more closely related to humans than they are to orangutans. Furthermore, they agree that chimps are more closely related to humans than chimps are to gorillas. In a family tree, chimps are our sisters, gorillas our cousins, and orangs are our cousins once removed.

TABLE 0.1. A Classification of Living Hominoids.

¹The hominini includes additional taxa not included here, for clarity.

WHAT MAKES AN APE?

Earlier I listed a few broad characteristics of biology and behavior that distinguish monkeys from apes and humans. The distinctions are important because in a way they approximate the path of evolution from our more monkey-like ancestors to our apelike ancestors to us. It is important to remember again that none of the living primates are our ancestors and that they have all evolved their own special characteristics. But we can see in monkeys today the enhanced intelligence, adaptability, and agility that was present in our common ancestor with them. We can see in the apes the further development of the brain and changes in body plan. Looking at the differences between moneys and hominoids helps us to retrace, as an approximation, the course of our evolution. So let’s look at this distinction in more detail.

First, there is the genetic evidence. We have known since the early part of the twentieth century that humans and apes are more similar to each other than to any other primate, when various organic molecules are compared. At first, comparisons among monkeys, apes, and humans involved proteins in the blood. Over a century ago researchers began to document similarities among Old World anthropoids to the exclusion of other primates.

In 1901 George Nuttall published a study in which he described a blood test to assess relationships among animals. Nuttall injected human blood serum (blood plasma from which the fibrogen, a clotting protein, has been removed) into rabbits. Blood collected from the rabbits was used to create an antiserum for human blood, that is, rabbit plasma with antibodies to human blood serum, produced by the rabbits as a natural immune reaction to human blood. Now he had a substance that he could use to detect human or humanlike blood. When Nuttall mixed this antiserum specific to humans with the sera from hundreds of different animals, he found that almost none of the mixtures reacted. In other words, there was nothing in the sera of any of these other animals that the human serum antibodies would react with—except monkey sera. The antibodies to human serum recognized something in the monkey sera.

Nuttall followed up this work in 1904 with a monograph entitled Blood immunity and blood relationship: a demonstration of certain blood-relationships amongst animals by means of the precipitin test for blood. He concluded that humans share a close relationship with the great apes and that the next closest relatives, in order, were Old World monkeys, New World monkeys, and prosimians (lemurs, lorises, galagos, and tarsiers). This was really a remarkable conclusion for the time because it is essentially what we think today. Nuttall even suggested that, given the difficulty of finding informative fossils, molecular techniques might be the best way to classify species and determine their evolutionary relationships.

Whether relationships among species can be determined by morphology (which is essentially all we have for fossils) or whether genes are the only reliable source of information is a debate that rages on today. As a paleoanthropologist, I advocate using morphology, as well as molecules, to help unravel the mysteries of ape and human origins, and I think that most of my colleagues would agree—even the molecular systematists who use DNA to reconstruct the tree of life.

As techniques grew more refined, it became possible to begin to differentiate among the Old World anthropoids. In the 1960s, researchers first proposed, based on molecular evidence, that humans are specifically related to African apes (this of course had been concluded much earlier by Darwin, and especially by Huxley, from morphological evidence).

Today it is possible to make detailed comparisons among organisms based on the actual sequence of base pairs in their DNA. The vast majority of analyses comparing the DNA sequence of humans with those of other primates yield the same results. Humans share a most recent ancestor with chimpanzees and bonobos. Gorillas are next most closely related to humans and chimps, and orangs are the next group out. Gibbons and siamangs are the so-called lesser apes, the living hominoids that first branched from the line leading to the great apes and humans.

Let’s put this in context.

Figure 0.2 is a diagram called a cladogram, which shows a nested set of relationships. It depicts the order of branching events but not direct ancestor-descendent relationships. You should read the cladogram as follows: New World monkeys branch off from the line leading to Old World anthropoids (catarrhines) and each goes on their own evolutionary path. This means that all New World monkeys have the same evolutionary relationship to all catarrhines. While this may seem counterintuitive (after all, we speak of both Old and New World monkeys), it is in fact documented by many lines of evidence, both morphological and molecular. So, New World monkeys as a group all have the same relationship with catarrhines (Old World monkeys, ape, and humans.) Both groups branched off from each other. Although capuchin monkeys from South America look a lot like vervet monkeys from Africa, they are in fact no more closely related to vervets than they are to Elvis (or any other hominoid). Vervets are more closely related to Elvis and all other hominoids, as well as to all other Old World monkeys, than they are to capuchins and all other New World monkeys. Get it? If not, have another look. It takes a while to properly understand a cladogram.

FIGURE 0.2. Cladogram of living anthropoids.

The next branching event separates the Old World monkeys from the apes (including humans). Again, this means that all Old World monkeys have the same evolutionary relationship to all hominoids. Although they may look to the lay person more like Old World monkeys than like humans, gibbons (hylobatids) are in fact more closely related to humans than they are to Old World monkeys.

And so it goes down the line. Gibbons branch off from the common ancestor of great apes and humans, and within this group, orangs diverge from African apes and humans, and finally gorillas diverge from chimps and humans. Despite the behavioral and morphological similarities that exist today among the African apes, chimpanzees and bonobos are more closely related to humans than they are to gorillas. This fact is reflected in the extreme similarity of the DNA of chimpanzees and humans, in the chimp-like anatomy of our ancestors, and in details of anatomy shared by humans, especially fossil humans, and chimps today (from now on, when I say chimps or chimpanzees, I am including bonobos as well.) The reason that chimps and gorillas look more similar to one another than chimps resemble humans, even though chimps and humans are more closely related to one another, is that chimps and gorillas share primitive characters that humans have lost. Our earliest fossil ancestors looked much more like chimpanzees than we do today.

Let me return to the cladogram and the science behind it, cladistics. Why is it that evolutionary biologists do not simply rely on overall similarity in reaching their conclusions about evolutionary relationships? After all,