September 29, 2010

The latest chapter of human evolution begins with the emergence of Homo sapiens. The anatomy of Homo sapiens is unique among hominin species and appears first in East Africa, dating to roughly 160 thousand years ago (ka). These unique features—including changes in the skull and postcranial skeleton (skeleton minus skull)—suggest changes in brain size and architecture and an adaptation to tropical environments. These anatomical changes are linked to cognitive and behavioral changes that are equally unique among hominin species. In particular, the archaeological evidence of behaviors thought to be unique to Homo sapiens, which appear first in Africa around 170 ka, highlight the importance of symbolism, complex cognitive behaviors, and a broad subsistence strategy (the strategy used to procure food). The evolution of Homosapiens is vitally important to defining our species in the broader context of human evolution and also has key to understanding the human condition, past and present.

Many of the unique features of Homo sapiens are found in the skull. Brain size estimates suggest that the brain of Homo sapiens was larger than in other hominin species. The average cranial capacity of Homo sapiens is roughly 1300 cubic centimeters, making the brains of this species absolutely smaller than those of Homo neanderthalensis. However, due to its gracile (small and lightly built) postcranial skeleton (see below), the brain of H. sapiens was larger relative to body size than that of H. neanderthalensis. The skull itself is much taller than earlier hominin species and the skull is therefore much shorter (from front to back) in relation to its height. The widest point of the Homosapiens skull is toward the top of the skull and the sides of the skull are nearly vertical. Together with the extreme height of the skull, these features give the Homo sapiens skull a house-shaped (i.e. pentagon with straight walls) appearance when viewed from behind. The skull of Homo sapiens also generally lacks evidence of being strongly built (e.g. it lacks the large browridges and bony prominences seen in Homoneanderthalensis and Homo heidelbergensis ). However, the earliest representatives of the species maintain a fairly rugged appearance. For instance, the browridges of early H. sapiens are fairly large, but differ in shape from those of H. neanderthalensis and H. heidelbergensis. The face of Homo sapiens is much smaller than those of earlier hominin species. In addition the face is “tucked under” the braincase—i.e. the face is located entirely below (rather than in front of) the front part of the brain. The face of this species also bears a deep indentation (the canine fossa) below each of the orbits. The teeth and mandible (lower jaw) are smaller than in earlier species and the mandible bears a prominent mental prominence (chin), which is not seen in any other hominin species.

Other unique features of the Homo sapiens skeleton are found in the postcranial skeleton. The postcranial skeleton is very lightly built relative to earlier species and lacks the adaptations to cold environments found in Homo neanderthalensis —e.g. thick, shortened limb bones and wide rib cages and hips. These features of the postcranial skeleton are thought to reflect the fact that Homo sapiens evolved in tropical, African environments before migrating across the globe. In these tropical environments, long and lean body proportions were favored because they maximized surface area (and therefore maximizing heat dissipation) while maintaining the same body mass.

Perhaps the most intriguing features found in Homo sapiens are behavioral traits. These traits, which can be discerned from the archaeological record, represent behavioral strategies that are not seen in any other hominin species. For instance, Homo sapiens is the first hominin species to hunt a large range of prey animals. Unlike earlier species, Homo sapiens developed hunting strategies that allowed them to hunt large, medium, and small mammals, as well as fish and shellfish. The stone tools that were made by Homo sapiens reflect this broader range of prey. In particular, Homo sapiens produced tools that ranged greatly in size and included very small artifacts used for hunting smaller game. The stone tools were also more diverse, reflecting a subsistence strategy that included many different types of animals of varying sizes. In Europe, lithic assemblages (the total sample of lithic artifacts from a particular site) associated with Homo sapiens also include blades (long, narrow artifacts defined as being at least twice as long as they are wide), an artifact rarely seen in lithic assemblages associated with Homoneanderthalensis. In addition, Homo sapiens was the first species to frequently produce tools made from bone and developed new technologies for making composite tools (tools that comprise different parts that are combined to form a tool). Homo sapiens ranged much longer distances to acquire raw materials for making stone tools than earlier species; this is often used as evidence that individual Homo sapiens had much larger ranges than other species. It is important to note that at some sites in western Asia, Homo sapiens are also found associated with tools that are typical of those made by Neandertals in Europe and in western Asia.

Other archaeological evidence strongly suggests that Homo sapiens was the first species to exhibit widespread use of symbolic behavior. For example, early Homo sapiens produced cave art, statues, shell beads, incised ochre, pendants, and figurines. While some authors contend that these items are also found at some Homo neanderthalensis sites, this opinion is contended and, at most, these items are found in much lower abundance at H. neanderthalensis sites. These traits suggest that Homo sapiens had a greater capacity for abstract, symbolic thinking than any earlier species. Based on the anatomy of the upper part of the vertebral column and associated structures, it is also highly likely that Homo sapiens was the first species that was capable of spoken language, but the evidence in support of this claim is scant.

The archaeological indicators of the modern behaviors associated with Homo sapiens do not show up at the same time across the range of the species. The first good evidence for these modern behaviors comes from Africa, starting over 150 ka. Different aspects of these behaviors showed up in a piecemeal fashion in Africa between 150 and 40 ka, when many of these features show up in concert at African Homo sapiens sites. In Europe, all of these features seem to appear at the same time. This is likely due to the fact that Homo sapiens originated in Africa and did not spread into Europe until almost 100 thousand years later. This hypothesis, however, is contended (see below).

The evolutionary relationship of Homo sapiens to other hominin species is very controversial. Most scientists believe that Homo sapiens evolved from populations of Homo heidelbergensi s in Africa between 150 and 200 ka. Following this evolution, the species spread throughout Africa, replacing local populations of H. heidelbergensis. Around 40-50 ka, Homo sapiens are thought to have migrated out of Africa replacing all populations of Homo neanderthalensi s in Europe and Homo heidelbergensi s elsewhere. The evidence for this position comes from both the fossil record and from studies of modern DNA. As mentioned above, the earliest undisputed fossil remains of Homo sapiens have been found in Africa, strongly suggesting that it was on this continent that the species first evolved. Genetic studies demonstrate that modern African populations are much more diverse and older than populations in other parts of the world. This evidence strongly suggest that Homo sapiens have been evolving longer in Africa than in other parts of the world, which, in turn, suggests that the origin of the species occurred in Africa and subsequently spread throughout the world. Adherents to this position (the so-called “Out of Africa/Replacement Model”) generally consider that Homo sapiens and Homo neanderthalensis were separate species and that little to no interbreeding occurred when the species encountered each other in Europe.

A minority of scholars maintains, instead, that H. neanderthalensis is a subspecies of Homo sapiens. According to this viewpoint (the so-called “Multiregional Model”), populations of “archaic” Homo sapiens (including Homo heidelbergensis and Homo neanderthalensis ) evolved into Homo sapiens in each region of the Old World. These scientists believe that all archaic populations were connected by gene flow (the sharing of genetic material due to interbreeding) from at least one million years ago onward. Thus, to these workers, although interbreeding was more frequent between populations within a region than between them, all of these populations represent a single, evolving species. This situation permitted the evolution of certain regional characteristics that distinguished populations in different regions, while still allowing universally favorable traits to spread across regions by gene flow. Scholars that adhere to this position point to purported anatomical continuity in all of the regions of the world from one million years ago to the present. For example, these authors claim that traits reminiscent of Homoneanderthalensis can be found in modern Europeans. Supporters of this idea believe that interbreeding between Homo neanderthalensi s and Homo sapiens was more widespread than is traditionally accepted and that Homo neanderthalensis made an important genetic contribution to living Homo sapiens .

Recent genetic research demonstrates there could be an intermediate position between these two strongly held ideas. This research, which was the result of the complete sequencing of the Homo neanderthalensis genome, suggests that that interbreeding between Homo neanderthalensis and Homo sapiens was rare, but larger than previously thought. Specifically, this research shows that the rate of interbreeding was between 1 and 4 percent. That is, between 1 and 4 percent of Homo sapiens breeding was with Neanderthals. This finding corroborates the ideas espoused by many scientists working in the field that neither of the two traditional models adequately represents the complexity of the interaction between Homo sapiens and Homo neanderthalensis. That is, Homo neanderthalensis and Homo sapiens were able to interbreed, but interbreeding was rather rare. Whether the amount of interbreeding constitutes the fact that Homo neanderthalensis made an important genetic contribution to living Homo sapiens is, more or less, a semantic matter, depending on how one defines an “important” genetic contribution. In light of this. questions about whether or not Homo sapiens and Homo neanderthalensis are different species or not is rather unimportant as it depends on how one chooses to define species—i.e. by the ability/inability to interbreed or by how evolutionarily distinct these two groups were.

What is clear from the European fossil record is the anatomy characteristic of Homo neanderthalensis disappeared rather rapidly between 50 and 30 ka and that this disappearance coincides with the migration of Homo sapiens into Europe from Africa. Thus, questions about what exactly caused that disappearance are crucially important and stridently contended. There is no evidence of warfare in Europe (or elsewhere) at this time. Therefore, the notion that Homo sapien s actively exterminated Homo neanderthalensis is difficult to maintain. Many scholars believe that the sophisticated symbolic and cognitive capabilities of Homo sapiens combined with their adaptation for hunting a large range of diverse prey allowed them to outcompete Homo neanderthalensis. This point of view is particularly relevant in light of the dramatically shifting climate that was witnessed in Europe around the time that Homo sapien s arrived. That is, Homo sapiens ’ broad subsistence strategy and ability to cognitively adapt to difficult environments may have allowed them to spread quickly and widely during times when the climate shifted dramatically. Homo neanderthalensis. on the other hand, may have had difficulty dealing with these drastic shifts in climate, due to their focus on hunting large game animals. Other researchers suggest Homo sapiens had a demographic, rather than a technological, edge on Homo neanderthalensis. These scholars suggest Homo sapiens maintained larger and more numerous populations and that, over time, the sheer numbers of Homo sapiens simply swamped out Homo neanderthalensis populations, which are thought to be smaller, more spread out, and less numerous. Of course, it is very possible that both of these factors (i.e. technology and demography) acted in concert, resulting in the disappearance of the Neanderthals .