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Human and Mammalian Evolution: Is There a Difference?

John de Vos and Jelle W. F. Reumer

Introduction

When God created the world, he did so in a succession of different steps. The creation of animals was one such step. The creation of mankind was another one. Ever since, mankind has been considered (i.e., has considered itself) not to be part of the animal kingdom. This notion—that Homo sapiens is a species next to, above, or outside the mammalian world—has long perverted science. Ernst Haeckel’s famous “Stammbaum des Menschen/Pedigree of Man,” published in 1874, shows “man” in the highest branch of the tree, above the rest of the living world, although part of the apes.

A favorite saying among mammalian paleontologists is the following: There are three classes of mammal paleontologists. The lowest one consists of those who study herbivores; that is, rodents and other small mammals, artiodactyls (bovids, cervids), and perissodactyls (horses). Although the fossil record of herbivores is vast, scientific output is unimpressive and, in addition, specimens are stored in cheap cardboard boxes. Higher up in the hierarchy are those who study carnivores. Although these paleontologists have less material to work with (because their subjects are higher up in the ecological food chain and thus less numerous), they produce more articles. Their fossils are carefully wrapped and stored in plastic boxes. High above these paleontologists are those who study fossil hominids. Although the human fossil record pales in the face of the fossil record of mammals in general, these paleontologists generate a disproportionate number of publications in high-ranking journals with the highest impact factors. Human fossils are stored in fireproof safes, often have nicknames (e.g., Lucy, Turkana boy, Hobbit, Little Foot), and are treated like icons. It is often easier to gain access to Fort Knox or Buckingham Palace than to see these specimens, let alone touch and study them. Those paleontologists who study or do analyses on fossil hominids and who consider theirs a separate profession are referred to as paleoanthropologists. This distinction reflects the general opinion that paleontology and paleoanthropology are different scientific enterprises. It is this misunderstanding that we discuss here.

We are surprised by the ongoing debate about whether or not the study of human evolution, and of Homo sapiens in particular, is part of evolutionary science. To us, as evolutionary biologists, it seems obvious that human evolution and mammalian evolution are inseparable and identical processes. Why should paleoanthropology be a separate science instead of part of mammalian paleontology? If human evolution does not differ from the evolution of other mammalian groups—and we believe it does not differ—then identical environmental factors may lead to identical adaptational responses, that is, to convergences. Can these be found?

We intend to answer this intriguing question by briefly discussing two historical examples: (1) the footprint trails of a tridactyl horse and a hominid from the Tanzanian site of Laetoli G and (2) the discovery of pygmy forms, such as dwarf elephants (from e.g., Malta, Sicily, Crete, and Flores), and dwarf hippopotamuses and other artiodactyls from islands of the Mediterranean and the Indonesian Archipelago, in relation to a small hominin from Flores (Homo floresiensis). We conclude with (3) a hypothesis about the pelage of Homo neanderthalensis in comparison with the woolly mammoth and woolly rhino as adaptations to the climatic circumstances on the Late Pleistocene mammoth steppe.

Man and Horse

In 1976, Mary Leakey discovered a rich locality with footprints that was excavated in 1978 and published the next year as “the Laetoli Footprints.” The presence of prints made by hominids amidst a rich mammalian ichnofauna was groundbreaking. At the site, three hominid trails are intersected by two Hipparion trails. Since then, numerous articles have been published on the Laetoli G hominid footprints (see De Vos et al. 1998 for an overview), but only few on the two Hipparion trails (Renders 1984; Renders and Sondaar 1987). Nevertheless, in spite of the lopsidedness of scientific and public attention, the footprints of hominid and equid provide information about the locomotion not only of the first bipedal hominids, but also of the tridactyl horse, and, together with preserved footprints of other species, ultimately about the Pliocene ecosystem in Eastern Africa.

The evolution of horses proceeded from the Eocene (ca. 55 Ma), with small, low-crowned, browsing forest-dwelling animals, to modern large, high-crowned grazers living on the grassy plains of the American prairies, the Eurasian steppes, and the African savannahs. Adaptation toward a life beyond the forests and woodlands led to long-legged horses with only one toe per extremity (Franzen 2010). This development accelerated in the Miocene when grassland ecosystems increased. The one-toed genus Equus appeared first in North America during the Pliocene, ca. 2–3 Ma ago, and subsequently spread to the Old World. With high-crowned teeth, a digestive system based on caecum fermentation, and long and sturdy legs suited for running, Equus is uniquely adapted to living on grassland (Franzen 2010). The Key Evolutionary Innovations (KEIs) involved in horse evolution are (1) lengthening of the legs, (2) an increase in hypsodonty and enamel wrinkling, and (3) development of a larger brain in relation to increasing socialization (Edinger 1948).

Antelopes (Bovidae) underwent a rapid increase in biodiversity in conjunction with climatic cooling and a consequent increase in grassland ecosystems (Vrba 1995). Especially from the Early Pliocene after 4 Ma ago and until the Middle Pleistocene, the number of African antelopes soared. Indeed, the increase in taxic diversity between 2.7 Ma and 2.5 Ma has been counted as 37 FADs (First Appearance Dates, Vrba 1995). Also in antelope evolution, KEIs involve lengthening of the legs, dental evolution, and increasing socialization (i.e., living in a herd structure). Further, this is precisely the period in time in which there was a major increase in hominids (especially within Australopithecus) and in which obligate hominid bipedalism emerged. We doubt that this is a mere coincidence.

Hominids are primates. “Primates of modern aspect” appear in the fossil record ca. 55 Ma, and striding (= modern) bipedal hominids (e.g., Turkana boy) after ca. 2 Ma ago in Africa. Current consensus is that modern-type bipedalism is an adaptation for living in an open environment, which resulted from the demise of African forests during the Late Pliocene/Early Pleistocene (De Vos et al. 1998). Recently, Uno et al. (2016) provided new evidence of an increase in grassland ecosystems, from which they concluded that “the biomarker vegetation record suggests that the increase in open, C4 grassland ecosystems over the last 10 Ma may have operated as a selection pressure for traits and behaviors in Homo such as bipedalism, flexible diets, and complex social structure” (Uno et al. 2016, p. 6355). Thus, both the evolution of human bipedalism and erect posture on the one hand, and of the long-legged running gait in horses on the other, are the result of Miocene-through-Pleistocene climate change in conjunction with the reduction of forest ecosystems and increase of open habitat. Both evolutionary processes are rooted in similar circumstances. The anatomical differences between hominids and equids can be explained by the different points of departure: that is, from a knuckle-walking plantigrade ape and a quadrupedal digitigrade equid, respectively. Yet, from an evolutionary biological perspective both processes are identical. KEIs involved in human evolution are the upright posture/bipedalism, dental evolution (in this case, among other, a reduction in canine size), and an increase in brain capacity in relation to greater dexterity and socialization. Humans, antelopes, and horses are mammals that adapted to a new environment, and their evolution reflects their convergences.

Humans and Island Dwarfs

For over a century paleontologists have studied island fossil vertebrates, resulting in a wealth of data from islands in the Mediterranean, the Philippines, Indonesia, the Californian Channel Islands, and many other islands and archipelagos (see Van der Geer et al. 2010 for an extensive overview). Although the mechanisms leading to observed phenomena remain unclear, these studies have given rise to what is called the “Island Rule.” That is, in general, small mammals (shrews, hedgehogs, rodents, leporids) become larger when isolated on islands, and large mammals (elephantids, hippopotamids, bovids, cervids) become smaller. A notorious example is the dwarf elephant Elephas falconeri from Spinagallo Cave, Sicily, with a shoulder height of only 90 cm in adult females, and 1.3 m in adult males (Van der Geer et al. 2010). Similar examples abound from islands around the globe. We now know, to list a few examples, of dwarf elephantids (genera Elephas, Mammuthus, Stegodon) from Sicily, Malta, Crete, Santa Rosa (and adjacent Channel Islands), Java, and Flores; of dwarf bovids from Mallorca, Menorca, and the Philippines; of dwarf cervids from Crete, Karpathos, the Ryu-Kyu Islands; and of dwarf hippopotamids from Crete, Malta, and Cyprus. The dwarfing of larger mammals appears to be an adaptation to insular circumstances, especially the absence of large mammalian predators (carnivores) and of the necessity to be large in order to escape from them. An important factor here is the ratio of body surface-to-volume: the smaller a large mammal becomes, the smaller is the risk of metabolic overheating. It is also important to note that size reduction can occur quite rapidly (Van der Geer et al. 2010), which is a fact of which Hawks (2016) was unaware when he wrote: “I’m very surprised to see paleoanthropologists in the press commenting that the dwarfing of Homo floresiensis was very rapid.” (Hawks 2016, final paragraph). In addition to dwarfism, the development of limb shortening and of low-gear locomotion is another island adaptation seen in larger mammals (Sondaar 1977; Van der Geer et al. 2010; Van Heteren 2012).

Although until fairly recently one might have wondered if humans would be an exception to the Island Rule, the possibility emerged with the discovery of the remains of a Late Pleistocene hominid on the Indonesian island of Flores (Morwood et al. 2004; Morwood and Van Oosterzee 2007; see also Van den Bergh et al. 2016). Claims of microcephaly notwithstanding, the specimens are more reasonably seen as evidence of island dwarfing and of a separate species. More recently, a possible second example of a small hominid was discovered in Callao Cave on the island of Luzon (Philippines; Mijares et al. 2010).

The adaptations seen in larger island mammals can be summarily pigeon-holed as “dwarfism,” but that would be too simplistic. Island adaptations comprise a set of characteristics, of which dwarfism is only one. They also include relative limb shortening, development of low-gear locomotion, relative decrease in brain size, and the emergence of specific dental features, such as reduction in number of molars, hypsodonty, hypselodont incisors, and/or changes in occlusal ridge patterning (Van der Geer et al. 2010). Altogether, such changes often blur the picture of the descent of the island taxa from their mainland ancestors.

Homo floresiensis is now considered to belong to a H. erectus clade (Zeitoun et al. 2016), and perhaps even a descendant of H. erectus, which is known from Indonesia from at least the early Middle Pleistocene (Dubois 1894; Van Heteren 2012; Van den Bergh et al. 2016). Among other features, H. floresiensis had short limbs, low-gear locomotion, and a small brain. In short, there is no morphological difference between island elephantids, bovids, cervids and hippopotamids and island hominids (i.e., H. floresiensis). Hominids are large mammals, and much like other large mammals, they dwarf and adapt similarly to insular circumstances. It is another example of a convergence.

Woolly “Man”

During the Late Pleistocene, the prevailing ecosystem in much of Eurasia was the so-called Mammoth Steppe (Guthrie 1982), which was a highly productive ecosystem with grasses, herbs, and low shrubs, and where a fauna called the Mammuthus-Coelodonta Faunal Complex thrived (hereafter MCFC; Kahlke 1999). The most common elements of this MCFC are the two eponymic taxa, the woolly mammoth (Mammuthus primigenius) and the woolly rhinoceros (Coelodonta antiquitatis), as well as various other species that sported a woolly pelage, such as the musk oxen (Ovibos moschatus) and the cave bear (Ursus spelaeus). A woolly pelage was the typical adaptation to the prevailing cold and dry circumstances. Most of the larger fauna went extinct at the end of the Pleistocene as a combined result of climate change towards a warmer and more mesic climate, and the fragmentation of their natural habitat. Among the taxa that disappeared entirely or regionally by the onset of the Holocene, are the woolly mammoth, North American Mastodon, Eurasian woolly rhinoceros, musk oxen (extinct in Eurasia only), Eurasian giant deer (Megaloceros giganteus), North American Equus, and a suite of carnivores, such as Homotherium, North American Smilodon, the hyena (Crocuta crocuta), the cave lion (Felis spelaea), and the cave bear (Ursus spelaeus).

Another victim of the Late Pleistocene was Homo neanderthalensis, which was a large-brained hominid that lived in a broad geographic band across Europe and part of western Asia (e.g., Endicott et al. 2010). Since “Classic” European Neanderthals were adapted to the harsh circumstances of the Late Pleistocene, and, albeit in relatively low numbers, lived on the Mammoth Steppe as part of the MCFC, one may wonder about their physical appearance. Indeed, since so many MCFC species were provided with a woolly pelage (mammoth, rhino, musk oxen, cave bear), we ask why this (rather than hairlessness) should not also have been the case with H. neanderthalensis. Hence, we propose that H. neanderthalensis was clad in a relatively thick, red or brownish-coloured pelage. We base our hypothesis on the fact that humans are mammals and therefore subject to the same evolutionary processes, including adaptations to a cold and dry climate, as other (large) mammals. For, indeed, why should most (if not all) MCFC mammals have had a thick woolly covering, and H. neanderthalensis be the exception? And in this context, we might consider seriously the likelihood that woolly Neanderthals went extinct along with the other woolly mammals.

Conclusion

Homo sapiens is a mammal. The evolution of the genus Homo, and of hominids in general, is, therefore, a part of mammalian evolution. Evolutionary processes that shape other large mammals, whether artiodactyl, perissodactyl, proboscidean, or any order of mammal, are in principle the same as, and cannot differ from, the evolutionary processes that shape hominids. The results of evolution within different groups of large mammals reveal interesting convergences. Here we briefly describe two such convergences. First, the adaptation of a long-legged running posture in association with the increase of grass ecosystems in the later part of the Cenozoic. The evolution of horses and African antelopes are prime examples of such adaptations. In this regard, hominid upright stance and bipedalism can be seen as a convergent adaptation. Second, is the dwarfing of larger mammals that become isolated on islands. Not only is this evidenced in the existence of pygmy elephants, pygmy hippopotamuses, dwarfed deer, and dwarfed bovids, but dwarfing is also documented for hominids by Homo floresiensis. Finally, based on the fauna that was contemporaneous with Neanderthals, we propose a third convergence: that is, like other Late Pleistocene mammals from the mammoth steppe, Homo neanderthalensis was also endowed with a thick, woolly pelage.

Acknowledgments

We’d like to thank Jeffrey Schwartz and the KLI for the invitation and the splendid organization.

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