Long before bipedal hominins first evolved in Africa, more primitive primates had diverged from even more distant mammalian ancestors. The roots of the primate order go back to the early stages of the placental mammal radiation at least 65 mya. Thus, the earliest primates evolved from early and still primitive placental mammals.
The earliest primates date to the Paleocene (65–56 mya) and belong to a large and diverse group of primitive mammals called the plesiadapiforms. These very early primates have been controversial for several decades, with opinions varying as to whether they actually are primates or members of a closely related but different group of mammals. Recently discovered quite complete fossils, coming especially from Montana and Wyoming, have now more firmly placed these Paleocene animals as the earliest known primates.
Eocene Primates: Closer Connections to Living Primates
From the succeeding Eocene epoch (56– 33 mya), a vast number of fossil primates have been discovered and now total more than 200 recognized species. Unlike the available Paleocene forms, those from the Eocene display more clearly derived primate features. These fossils have been found at many sites in North America and Europe (which for most of the Eocene were still connected). In addition, more recent finds have shown that the radiation of Eocene primates extended to Asia and Africa. It’s important to recall that the landmasses that connect continents, as well as the water boundaries that separate them, have an obvious impact on the geographical distribution of all land animals, including primates.
The most complete early primate fossil ever found was announced in 2009 and, in honor of the 200th anniversary of Darwin’s birth, is called Darwinius . It comes from the Eocene Messel site in Germany, dates to 47 mya (during the Eocene), and is extraordinarily well preserved (Franzen et al., 2009). At the time of its announcement, it created a public sensation, although the find has a complex and somewhat peculiar history. Clearly, Darwinius was meant to make a big splash, and that it did; yet, virtually no other experts in early primate evolution have had an opportunity to see the original fossil, and thus the wide publicity it received. There is much to learn about the adaptations of this small Eocene primate, but it seems unlikely that it provides direct evidence of a close connection to us and other anthropoids as claimed. Indeed, it may not be particularly closely related to any living primate.
Regarding Eocene fossils, it’s certain that they were (1) primates, (2) widely distributed, and (3) mostly extinct by the end of the Eocene.
What is less certain is how any of them might be related to the living primates. Some of these forms were probably similar to and are potential ancestors of the lemurs and lorises. Others are probably related to tarsiers. By far, however, most of the Eocene primates (including Darwinius and its close relatives) don’t appear to have been ancestral to any later primate, and they became extinct before the end of the Eocene (around 33 mya). Nevertheless, some fossil finds from late in the Eocene have derived features (such as a dental comb) that link them to modern lemurs and lorises.
New evidence of Eocene : anthropoid origins has recently been discovered at a few sites in North Africa. The earliest of these African fossils go back to 50 mya, but the remains are very fragmentary. More conclusive evidence comes from Egypt and is well dated to 37 mya. At present, it looks likely that the earliest anthropoids first evolved in Africa.
Oligocene Primates: Anthropoid Connections
The Oligocene (33–23 mya) has yielded numerous additional fossil remains of several different early anthropoid species. Most of these are Old World anthropoids, all discovered at a single locality in Egypt, the Fayum. In addition, there are a few known bits from North and South America that relate only to the ancestry of New World monkeys. By the early Oligocene, continental drift had separated the New World (that is, the Americas) from the Old World (Africa and Eurasia). Some of the earliest Fayum species, nevertheless, may potentially be close to the ancestry of both Old and New World anthropoids. It’s been suggested that late in the Eocene or very early in the Oligocene, the first anthropoids (primitive “monkeys”) arose in Africa and later reached South America by “rafting” over the water separation on drifting chunks of vegetation. What we call “monkey,” then, may have a common Old World origin, but the ancestry of New and Old World monkeys was separate after about 35 mya. After this time, the closest evolutionary connections humans have are with other Old World anthropoids—that is, with Old World monkeys and apes.
The possible roots of anthropoid evolution are illustrated by different forms from the Fayum; one is the genus Apidium. Well known at the Fayum, Apidium is represented by several dozen jaws or partial dentitions as well as many postcranial remains. Owing to its primitive dental arrangement, some paleontologists have suggested that Apidium may lie near or even before the evolutionary divergence of Old and New World anthropoids. Because so much fossil material of teeth and limb bones of Apidium has been found, some informed speculation regarding diet and locomotor behavior is possible. It’s thought that this small, squirrel-sized primate ate mostly fruits and some seeds and was most likely an arboreal quadruped, adept at leaping and springing.
The other genus of importance from the Fayum is Aegyptopithecus. This genus is represented by several well preserved crania and abundant jaws and teeth. The largest of the Fayum anthropoids, Aegyptopithecus is roughly the size of a modern howler monkey (13 to 18 pounds; Fleagle, 1983) and is thought to have been a short-limbed, slow-moving arboreal quadruped. Aegyptopithecus is important because, better than any other known form, it bridges the gap between the Eocene fossils and the succeeding Miocene hominoids.
Nevertheless, Aegyptopithecus is a very primitive Old World anthropoid, with a small brain and long snout and not showing any derived features of either Old World monkeys or hominoids. Thus, it may be close to the ancestry of both major groups of living Old World anthropoids. Found in geological beds dating to 35–33 mya, Aegyptopithecus further suggests that the crucial evolutionary divergence of hominoids from other Old World anthropoids occurred after this time.
Miocene Fossil Hominoids: Closer Connections to Apes and Humans
During the approximately 18 million years of the Miocene (23–5 mya), a great deal of evolutionary activity took place. In Africa, Asia, and Europe, a diverse and highly successful group of hominoids emerged . Indeed, there were many more kinds of hominoids from the Miocene than there are today (now represented by just a few ape species and humans). In fact, the Miocene could be called “the golden age of hominoids.” Many thousands of fossils have been found from dozens of sites scattered in eastern Africa, southern Africa, southwest Asia, into western and southern Europe, and extending into southern Asia and China.
During the Miocene, significant transformations relating to climate and repositioning of landmasses took place. By 23 mya, major continental locations approximated those of today (except that North and South America were separate). Nevertheless, the movements of South America and Australia farther away from Antarctica significantly altered ocean currents. Likewise, the continued collision between the South Asian Plate and southern Asia produced the Himalayan Plateau. Both of these geographical changes had significant impacts on the climate, and the early Miocene was considerably warmer than the preceding Oligocene. Moreover, by 19 mya, the Arabian Plate (which had been separate) “docked” with northeastern Africa. As a result, migrations of animals from Africa directly into southwest Asia (and in the other direction as well) became possible. Among the earliest transcontinental migrants (around 16 mya) were African hominoids that colonized both Europe and Asia at this time.
A problem arises in any attempt to simplify the complex evolutionary situation regarding Miocene hominoids. For example, for many years, paleontologists tended to think of these fossil forms as either “apelike” or “humanlike” and used modern examples as models. But as we have just noted, very few hominoids remain. Therefore, we should not hastily generalize from these few living forms to the much more diverse fossil forms; otherwise, we obscure the evolutionary uniqueness of these animals. In addition, we should not expect all fossil forms to be directly or even particularly closely related to living species. Indeed, we should expect the opposite; that is, most lines vanish without descendants. Over the last three decades, the Miocene fossil hominoid assemblage has been interpreted and reinterpreted. As more fossils are found, the evolutionary picture becomes more complicated. What’s more, most of the fossils haven’t been completely studied, so conclusions remain tenuous.
Given this uncertainty, it’s probably best, for the present, to group Miocene hominoids geographically:
- African forms (23–14 mya): known especially from western Kenya, these include quite generalized, and in many ways primitive, hominoids. The best-known genus is Proconsul. In fact, Proconsul isn’t much like an ape, and postcranially it more closely resembles a monkey. But there are some derived features of the teeth that link Proconsul to hominoids.
- European forms (16–11 mya): Known from widely scattered localities in France, Spain, Italy, Greece, Austria, Germany, and Hungary, most of these forms are quite derived. However, this is a varied and not well-understood group. The best known of these are placed in the genus Dryopithecus; the Hungarian and Greek fossils are usually assigned to other genera. The Greek fossils, called Ouranopithecus, date to 10–9 mya. Evolutionary relationships are uncertain; some researchers have suggested a link with the African ape–hominin group, but most primatologists think that these similarities result from homoplasy-convergency (Wood and Harrison, 2011).
- Asian forms (15–5 mya) :The largest and most varied group of Miocene fossil hominoids was geographically dispersed from Turkey through India/Pakistan and east to Lufeng, in southern China. The best-known genus is Sivapithecus (from Turkey and Pakistan), and fossil evidence indicates that most of these hominoids were highly derived.
- Four general points are certain concerning Miocene hominoid fossils: They are widespread geographically; they are numerous; they span essentially the entirety of the Miocene, with known remains dated between 23 and 6 mya; and at present, they are poorly understood. However, we can reasonably draw the following conclusions:
- These are hominoids—more closely related to the ape-human lineage than to Old World monkeys.
- They are mostly large-bodied hominoids, that is, more connected to the lineages of orangutans, gorillas, chimpanzees, and humans than to smaller-bodied apes (gibbons and siamangs).
- Most of the Miocene species thus far discovered are so derived that they are probably not ancestral to any living form.
- One lineage that appears well established is Sivapithecus from Turkey and Pakistan. Sivapithecus shows some highly derived facial features similar to the modern orangutan, suggesting a fairly close evolutionary connection.
- Evidence of definite hominins from the Miocene hasn’t yet been indisputably confirmed.
- However, exciting recent (and not fully studied) finds from Kenya, Ethiopia, and Chad (the latter dating as far back as 7–6 mya) suggest that hominins diverged sometime in the latter Miocene . As we shall see shortly, the most fundamental feature of the early hominins is the adaptation to bipedal locomotion. In addition, recently discovered Miocene remains of the first fossils linked closely to gorillas (Suwa et al., 2007) provide further support for a late Miocene divergence (about 10–7 mya) of our closest ape cousins from the hominin line. The only fossil chimpanzee so far discovered has a much later date of around 500,000 years ago (ya), long after the time that hominins split from African apes (McBrearty and Jablonksi, 2005).
Understanding our Direct Evolutionary Connections: What’s a Hominin?
The earliest evidence of hominins dates to the end of the Miocene and mainly includes dental and cranial pieces. But dental remains alone don’t describe the special features of hominins, and they certainly aren’t distinctive of the later stages of human evolution.
Modern humans, as well as our most immediate hominin ancestors, are distinguished from the great apes by more obvious features than tooth and jaw dimensions. For example, various scientists have pointed to such distinctive hominin characteristics as bipedal locomotion, large brain size, and toolmaking behavior as being significant (at some stage) in defining what makes a hominin a hominin.
It’s important to recognize that not all these characteristics developed simultaneously or at the same pace. In fact, over the last several million years of hominin evolution, quite a different pattern has been evident, in which each of the components (dentition, locomotion, brain size, and toolmaking) have developed at quite different rates. This pattern, in which physiological and behavioral systems evolve at different rates, is called mosaic evolution. The single most important defining characteristic for the entire course of hominin evolution is bipedal locomotion. In the earliest stages of hominin emergence, skeletal evidence indicating bipedal locomotion is the only truly reliable indicator that these fossils were indeed hominins. But in later stages of hominin evolution, other features, especially those relating to brain development and behavior, become highly significant.
What’s in a Name?
We refer to members of the human lineage as hominins ( name for members of the tribe Hominini). Most paleoanthropologists now prefer this terminology, since it more accurately reflects evolutionary relationships. The more traditional classification of hominoids isn’t as accurate and actually misrepresents key evolutionary relationships. In the last several years, detailed molecular evidence clearly shows that the great apes (traditionally classified as pongids and including orangutans, gorillas, chimpanzees, and bonobos) don’t make up a coherent evolutionary group sharing a single common ancestor.
Indeed, the molecular/genetic data indicate that the African great apes (gorillas, chimpanzees, and bonobos) are significantly more closely related to humans than is the orangutan. What’s more, at an even closer evolutionary level, we now know that chimpanzees and bonobos are yet more closely connected to humans than are gorillas. Hominoid classification has been significantly revised to show these more complete relationships, and two further taxonomic levels (subfamily and tribe) have been added.
We should mention a couple of important ramifications of this new classification. First, it further emphasizes the very close evolutionary connection of humans with African apes and most especially with chimpanzees and bonobos. Second, the term hominid, which has been used for decades to refer to our specific evolutionary lineage, has a quite different meaning in the revised classification; now it refers to all great apes and humans together. Unfortunately, during the period of transition to the newer classification scheme, confusion is bound to result. For this reason, we won’t use the term hominid in this book except where absolutely necessary. To avoid confusion, we’ll simply refer to the grouping of great apes and humans as “large bodied hominoids.”
Digging for Connections: Early Hominins from Africa
A variety of early hominins lived in Africa, and we’ll cover their comings and goings over a 5- million-year period, from at least 6 to 1 mya. It’s also important to keep in mind that these hominins were geographically widely distributed, with fossil discoveries coming from central, East, and South Africa. Paleoanthropologists generally agree that among these early African fossils, there were at least 6 different genera, which in turn comprised upward of 13 different species. Some of the earliest fossils thought by many researchers to be hominins are primitive in some ways and unusually derived in others. In fact, some paleoanthropologists remain unconvinced that they are really hominins.
Our primary focus will be to organize them by time and by major evolutionary trends. In so doing, we recognize three major groups:
▶ Pre-australopiths—the earliest and most primitive (possible) hominins (6.0+ – 4.4 mya) [Sahelanthropus chadensis , Orrorin , Ardipithecus]
▶ Australopiths—diverse forms, some more primitive, others highly derived (4.2 – 1.2 mya) [Australopithecus and Paranthropus]
▶ Early Homo—the first members of our genus (2.0+ – 1.4 mya
Seeing the Big Picture: Adaptive Patterns of Early African Hominins
As you are by now aware, there are several different African hominin genera and certainly lots of species. This, in itself, is interesting. Speciation was occurring quite frequently among the various lineages of early hominins— more frequently, in fact, than among later hominins. What explains this pattern?
Evidence has been accumulating at a furious pace in the last decade, but it’s still far from complete. What’s clear is that we’ll never have anything approaching a complete record of early hominin evolution, so some significant gaps will remain. After all, we’re able to discover hominins only in those special environmental contexts where fossilization was likely. All the other potential habitats they might have exploited are now invisible to us. Still, patterns are emerging from the fascinating data we do have. First, it appears that early hominin species (pre-australopiths, Australopithecus, Paranthropus, and early Homo) all had
restricted ranges. It’s therefore likely that each hominin species exploited a relatively small area and could easily have become separated from other populations of its own species. So genetic drift (and to some extent natural selection) could have led to rapid genetic divergence and eventual speciation. Second, most of these species appear to be at least partially tied to arboreal habitats, although there’s disagreement on this point regarding early Homo (see Wood and Collard, 1999b; Foley 2002).
Also, Paranthropus was probably somewhat less arboreal than Ardipithecus or Australopithecus. These very largetoothed hominins apparently concentrated on a diet of coarse, fibrous plant foods, such as roots. Exploiting such resources may have routinely taken these hominins farther away from the trees than their dentally more gracile— and perhaps more omnivorous— cousins.
Third, except for some early Homo individuals, there’s very little in the way of an evolutionary trend of increased body size or of markedly greater encephalization. Beginning with Sahelanthropus, brain size was no more than that in chimpanzees— although when controlling for body size, this earliest of all known hominins may have had a proportionately larger brain than any living ape. Close to 5 million years later (that is, the time of the last surviving australopith species), relative brain size increased by no more than 10 to 15 percent. Perhaps tied to this relative stasis in brain capacity, there’s no absolute association of any of these hominins with patterned stone tool manufacture. Although conclusions are becoming increasingly controversial, for the moment, early Homo appears to be a partial exception. This group shows both increased encephalization and numerous occurrences of potential association with stone tools (though at many of the sites, australopith fossils were also found).
Lastly, all of these early African hominins show an accelerated developmental pattern (similar to that seen in African apes), one quite different from the delayed developmental pattern characteristic of Homo sapiens (and our immediate precursors). This apelike development is also seen in some early Homo individuals (Wood and Collard, 1999a). Rates of development can be accurately reconstructed by examining dental growth markers (Bromage and Dean, 1985), and these data may provide a crucial window into understanding this early stage of hominin evolution.
These African hominin predecessors were rather small, able bipeds, but still closely tied to arboreal and/or climbing niches. They had fairly small brains and, compared to later Homo, matured rapidly. It would take a major evolutionary jump to push one of their descendants in a more human direction. For the next chapter in this more human saga, read on.
Questions:
- In what ways are the remains of Sahelanthropus and Ardipithecus primitive? Why do many paleoanthropologists classify these forms as hominins? How sure are we?
- Assume that you are in the laboratory analyzing the Lucy A. afarensis skeleton. You also have complete skeletons from a chimpanzee and a modern human. (a) Which parts of the Lucy skeleton are more similar to the chimpanzee? Which are more similar to the human? (b) Which parts of the Lucy skeleton are most informative?
- The oldest identified hominin tools are roughly 2.6 million years old, but evidence of hominin tool use is much older. Why did hominins become toolmakers?
- What is a phylogeny? Construct one for early hominins (7.0–1.0 mya). Make sure you can describewhat conclusions your schememakes. Also, try to defend it.