THE MILESTONE OF HUMAN EVOLUTION....

The emergence of Homo sapiens..............

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 Homo sapiens 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.

                                               Neandertal                           Homo sapiens
 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 Homo sapiens  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 Homo neanderthalensis 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 brain case 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.
 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 sapiens actively exterminated Homo neanderthalensis is difficult to maintain.  Many people 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 sapiens 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 .

People Today...
Are we genetically different from our Homo sapiens ancestors who lived 10-20,000 years ago?  The answer is almost certainly yes.  In fact, it is very likely that the rate of evolution for our species has continuously accelerated since the end of the last ice age, roughly 10,000 years ago.  This is mostly due to the fact that our human population has explosively grown and moved into new kinds of environments, including cities, where we have been subject to new natural selection pressures.  For instance, our larger and denser populations have made it far easier for contagious diseases, such as tuberculosis, small pox, the plague, and influenza to rapidly spread through communities and wreak havoc.  This has exerted strong selection for individuals who were fortunate to have immune systems that allowed them to survive.  There also has been a marked change in diet for most people since the end of the last ice age.  It is now less varied and predominantly vegetarian around the globe with a heavy dependence on foods made from cereal grains.  It is likely that the human species has been able to adapt to these and other new environmental pressures because it has acquired a steadily greater genetic diversity.  A larger population naturally has more mutations adding variation to its gene pool simply because there are more people.  This happens even if the mutation rate per person remains the same.  However, the mutation rate may have actually increased because we have been exposed to new kinds of man-made environmental pollution that can cause additional mutations.
It is not clear what all of the consequences of the environmental and behavioral changes for humans have been.  However, it does appear that the average human body size has become somewhat shorter over the last 10,000 years, and we have acquired widespread immunity to the more severe effects of some diseases such as measles and influenza.
Finally, can we say what direction human evolution will take in the future?  This is a fascinating question to consider but impossible to answer because of innumerable unknown factors.  Though, it is certain that we will continue to evolve until we reach the point of extinction.