Adaptation in Human Evolution

States of organismic phenotypes (an item of behavior, physiological process, or anatomical property) shaped by natural selection to perform a specific role. The evolutionary process of natural selection acting to shape, maintain, or modify such properties is also known as adaptation. The theory of adaptation is the evolutionary biological explanation for the design apparent in nature, whereby organisms appear to display a close fit to their environments. Adaptation is the central focus of Darwin’s original formulation of evolutionary theory and of most modern formulations of the evolutionary process.
Much remains to be learned about the process of adaptation. On the one hand, theorists since Darwin have argued that selection should constantly improve the quality of adaptations or modify adaptations to keep pace with changing environments. According to this view of adaptation, constant, gradual change should be the norm. On the other hand, many species remain stable in most of their characteristics for long periods of their history (the phenomenon of stasis), and thus it is assumed that natural selection lends stability and conserves adaptations for large portions of a species’ history. According to this “punctuational” view, adaptive change is relatively rare in evolution, is relatively rapid when it occurs, and is most often associated with speciation.

A Guide to Adapiformes Infraorder

Primate infraorder including the mainly Eocene Adapidae and their close relatives, as distinguished from the living Lemuriformes. Together, Adapiformes and Lemuriformes form the Strepsirhini. Adapiformes was erected by F.S.Szalay and E.Delson to distinguish a collection of primarily Eocene primates from more recent and supposedly descendant strepsirhines. Adapiformes here subsumes the super-family Adapoidea, which in turn contains the families Adapidae, Notharctidae, and perhaps Sivaladapidae. Adapoidea, when used previously, had included only the Holarctic family Notharctidae and the European family Adapidae and had been grouped with extant taxa in the infraorder Lemuri-formes. Some researchers have thought that Sivaladapis and other southern Asian Miocene forms could be related to adapids and distinguished as the family Sivaladapidae, but it seems that this concept combines unrelated taxa whose phyletic links are to different strepsirhine groups (Notharctidae and Lorisidae); the family is no longer recognized here.
Szalay and Delson suggested that the adapiforms could be distinguished from all lemuriforms because they lack the derived tooth comb that characterizes the latter group. Here, however, it is argued that the only feature that distinguishes Adapiformes as a group apart from extant strepsirhines is its members’ greater antiquity.

There are no morphological features peculiar to adapiforms that would attest to their monophyly: The lack of a tooth comb is an ancestral condition that does not unify adapiforms or any other group; it is not even clear that the mere presence of a tooth comb unites all lemuriforms to the exclusion of any “adapiform.”
Inasmuch as characteristics of Strepsirhini are based historically on aspects of softtissue
morphology, the phylogenetic association of any adapiform with extant taxa must
be based on fossilizable material. Traditionally, the association of adapiforms with extant
taxa rested primarily on the sharing by various notharctids, adapids, and lemurs of the
“lemurlike” bulla—i.e., an “inflated” auditory bulla whose lateral edge extends laterally
beyond the inferior margin of the tympanic ring (the “free” tympanic ring). Recent
studies of the wrist and ankle morphology of various extant primates and bones of these
regions attributed to Adapis, Leptadapis, Notharctus, Cantius, Caenopithecus, and
Smilodectes have concluded that certain features, while not uniting adapiforms as a group, are suggestive of the overall monophyly of “adapiform” and extant lemuriform taxa. These same studies of the postcranium, as well as earlier analyses based on craniodental morphology, came to the conclusion that Adapidae and Notharctidae, at least, were not sister taxa. Rather, the former taxon was more closely related to extant lemurs than was the latter.

To retain the overall pattern of primate phylogeny and classification laid out for this encyclopedia, Adapiformes is here utilized as a paraphyletic taxon. Genera previously included in a unitary family Adapidae have been allocated to the families Adapidae and Notharctidae or placed as possible adapiforms of uncertain relationship. Some of the latter have also been suggested as possible protoanthropoids and/or included in the adapid (or notharctid) subfamily Cercamoniinae (=?Protoadapinae).

Infraorder Adapiformes
Family indeterminate
†Caenopithecus
†Lushius
†Azibius
†Panobius
†Djebelemur
†Wailekia
†Rencunius
†Pondaungia
†Hoanghonius
†extinct

Phylogenetic Relationships of Adapidae Family

In addition to their ancientness (Middle-to-Late Eocene), adapids have been sought as potential ancestors of modern strepsirhines because of features that have been presumed to be primitive. Adapids lack a tooth comb of the sort seen in modern lemurs and lorises; they typically have a greater number of premolars (four as opposed to three in each quadrant of the jaw); and they have a “lemurlike” bulla, which, because it is similar to that in Lemur, was seen, almost by definition, as primitive. Aside from the occasional inconsis tency, such as having a fused mandibular symphysis, Adapis especially could fulfill the role of ancestor to the modern strepsirhines. Gregory even argued that dental similarities between the fossil form and the extant Malagasy lemur, Lepilemur, demonstrated the primitiveness among the living taxa of Lepilemur and thus the descent from Adapis of other lemurs via Lepilemur. Just over 50 years later, P.D.Gingerich thought the dental similarities were greater between Adapis and the extant Hapalemur and thus suggested that this genus, rather than Lepilemur, was the link between the extinct taxon and the other modern strepsirhines, a view not accepted here. In 1979, J.H.Schwartz and I.Tattersall turned the argument around and suggested that the distinctiveness of the compressed cusps and shearing crests of the molars of Adapis, as well as Hapalemur and Lepilemur, indicated that these taxa were closely related and specialized members of Strepsirhini, forming a separate clade; these authors included the Notharctus group in Adapidae. Subsequently Schwartz pointed out that there really are no features that would unite a Notharctus group with an Adapis group, and he and Tattersall presented dental and some cranial evidence suggesting a relationship between Adapidae, in the restricted sense of Adapis plus those few forms sharing derived characters with it, and a particular group of Malagasy primates, the indrioids.
During this latter review, Schwartz and Tattersall failed to discover any derived characters that would unite with Adapis those taxa traditionally placed into Adapidae. As Robinson had suggested about North American fossils included in the (primarily) Eocene family Omomyidae, it seemed that taxa had been placed in Adapidae because they were Eocene in age and European in location. An appraisal of the spectrum of so-called adapids led to the suggestion that some were actually related to Notharctus or Pelycodus, such as Cercamonius and Protoadapis, and Pronycticebus and Agerinia, respectively; others were linked to extant taxa, such as the fossil genus Huerzeleris to the living Malagasy primate, Phaner, and yet others were lorisoids of uncertain affinity, such as Anchomomys and Periconodon.
Adapidae seemed, therefore, to be a group of few members (Adapis and Leptadapis, as well as the recently proposed genera Simonsia and Paradapis) related to a small number of specialized extant primates. Pelycodus-also emerged as sharing some potential derived features with Notharctus, as well as others with Smilodectes. Although not contributing to a resolution of its relationships, this does indicate that Pelycodus could not have been ancestral to both a Notharctus group and an Adapis group. More recently, postcranial evidence has been brought to bear on the relationships of the Adapis group to the Notharctus group and of each of these groups to extant taxa. Studies by K.C.Beard and colleagues of wrist and ankle bones attributed to Adapis, Notharctus, Cantius, and Smilodectes indicated that there were distinct differences between Adapis and the three taxa representative of the Notharctus group. In a comparison with a diversity of extant primates, Beard et al. concluded that the Adapis group was more closely related to extant lemurs than to the Notharctus group because Adapis shared with extant lemurs a unique articulation between the ulna and the small pisiform bone of the wrist. This feature is not found in Smilodectes (the only taxon of the Notharctus group for which the appropriate bones are known) and is apparently not characteristic of the anthropoid primates analyzed. Thus, Beard et al. concluded that certain aspects of wrist morphology corroborated the interpretation based on craniodental features: The Notharctus group and the Adapis group are not sister taxa. Beard et al. did not, however, find support for the suggestion that Adapis may be closely related to only a few of the extant lemurs. Rather, these authors argued that another feature of the wrist—an os centrale that overlaps the capitate and makes contact with the hamate—is found uniquely in extant lemurs to the exclusion of Adapis. Although research being conducted by Schwartz and Yamada indicates that some features of wrist and ankle morphology require further documentation, it is apparent that the traditional phylogenetic and systematic schema involving Adapidae are in need of revision.

Family Adapidae
Subfamily Adapinae
†Adapis
†Leptadapis
†Simonsia
†Paradapis
†Cryptadapis
†Alsatia
†extinct

A Guide to Adapidae Family

Extinct primate family that has come to include a plethora of European Eocene primates ranging in size from as small as a mouse (Anchomomys) to as big as a large cat (Leptadapis). According to studies of body size and molar shearing-crest development, the larger forms (Adapis, Leptadapis, Caenopithecus, Protoadapis, Europolemur) were probably folivorous, whereas the smaller forms (e.g., Periconodon, Anchomomys, Microadapis, Agerina), and possibly Pronycticebus as well, were probably insectivorous, with the latter three taxa perhaps also including fruit in their diet.
Although Adapidae is associated here with Notharctidae, it is only within the former group that the ancestry of modern strepsirhines has traditionally been sought.
The genus Adapis, which gives its name to the family Adapidae as well as to taxa of other ranks, was described in 1821 by the French paleontologist G.Cuvier, who thought it might be either a pachyderm or an artiodactyl. Despite this “false start,” Adapis claims the distinction of being the first fossil primate to be studied. Since its discovery, Adapis has become one of the best known of all European fossil primates: It is a particularly dominant mammal in collections from the limestone deposits of the Franco-Belgian Basin. The genus Leptadapis, the largest of the adapids, used to be included as a species of Adapis (A. magnus), but the genus Adapis is now reserved for the original form, A. parisiensis, and perhaps one other species of comparable size. In 1912, the Swiss paleontologist H.G.Stehlin published a monographic study of Adapis (including “Adapis” magnus). In comparing it especially with the North American Notharctus, he concluded that, while the Old and New World taxa may somehow be related, differences warranted distinction at the family level between the groups they represented.
This matter was addressed by the American paleontologist W.K.Gregory in his 1920 work on Notharctus, in which he argued that differences between Adapis and Notharctus in skull shape and particularly in dental elaboration (more in the latter taxon), while real, were no less profound than differences that existed among miacids, an assemblage of extinct but diverse carnivores that all paleontologists seemed to agree belonged in the same family. Thus, Gregory concluded that it was appropriate to group the European taxa in the subfamily Adapinae and the North American forms in Notharctinae and to subsume both in the family Adapidae. The common ancestor of both adapid subfamilies was taken to be the Early Eocene Pelycodus (then known only from North America but subsequently also from Europe), from which Gregory believed that both the geologically younger Adapis and Notharctus could have evolved.
This basic phylogenetic scheme was not altered in the ensuing few decades, but largely through the studies of P. Robinson and C.L.Gazin in the 1950s, Stehlin’s suggestion that the European and the North American taxa should be separated at the family level was revived. Thus, two alternative classificatory schemes have been applied to the family Adapidae: most recently, E.L.Simons and F.S.Szalay and E. Delson have preferred Gregory’s subfamily divisions, while in this volume, for example, the distinctiveness of the two groups is maintained at the family level.

A Guide to Acheulean Artifacts

Early Paleolithic industry characterized by handaxes and similar types of modified stone tools. Acheulean artifact assemblages are known from ca. 1.5 to 0.2Ma and span Africa, Europe, and Asia. Based originally on numerous handaxes discovered at the site of St. Acheul (France), the term Acheulean is applied to stone assemblages with large bifacially flaked, ovoid tools. In an artifact assemblage, such tools must be abundant and/or finely made for the term to apply. In Africa, where the oldest Acheulean occurrences are known, handaxes and similar tools, such as cleavers and picks, are grouped under the term bifaces. Acheulean bifaces are highly standardized compared with flaked pieces of earlier non-Acheulean industries. It has been suggested that Acheulean sites in Africa are those where 40 percent or more of the intentionally flaked stones (i.e., tools or cores) are bifaces. However, sites where bifaces are fewer but are flaked carefully and symmetrically are also called Acheulean. In the view of some archaeologists, these criteria distinguish the Acheulean from other industries containing rare and crudely flaked bifaces, such as the Developed Oldowan or Clactonian. Still other researchers claim that, since the Acheulean is a tradition of tool manufacture involving the production of bifaces, any assemblage with such tools represents the Acheulean.
Preceded by the Oldowan and related core-flake tool kits, the Acheulean may have originated by gradual transition in the degree to which oval-shaped cobbles were flaked (chopper to protohandaxe to handaxe). Particularly in Europe, the idea of gradual refinement in tool manufacture from pre-Acheulean to Acheulean and throughout the Early Paleolithic period has been thought to involve a shift from using hammerstones in tool manufacture to “soft” hammers, such as bone or antler, which permit greater control over the transmission of force needed to remove a flake. It was suggested by G.L.Isaac, however, that the ability to remove large flakes (greater than 10cm in length) was essential to the emergence of the Acheulean in Africa. This ability may have represented a threshold in tool manufacture, rapidly exploited as a starting point in the manufacture of
bifaces. The rough oval shape of early bifaces is a natural extension of the original form of large flakes regardless of whether they had been further shaped intentionally into preconceived tools or simply used as cores for efficient production of sharp flakes. In Early Acheulean assemblages, such as those at Olduvai Gorge (Tanzania), it is nonetheless true that bifaces were sometimes made on cobbles and also on flakes smaller than 10cm. Thus, it is still unclear whether the manufacture of Acheulean bifaces came about by gradual refinement in the flaking of cobbles or by a technical refinement in the ability to produce large flakes.
Acheulean bifaces represent the distinctive product of early human technology during a period exceeding 1Myr. Studies of sequences of sites from individual localities, such as Olorgesailie (Kenya), have shown that handaxe manufacture and the overall makeup of Acheulean assemblages are marked by conservative, nonprogressive variation over hundreds of thousands of years. Moreover, examples of bifaces from Africa, Europe, and Asia are remarkably similar to one another, despite the great distances between localities. Biface forms nevertheless did undergo refinement over the time span of the Acheulean. In the early Acheulean, handaxes and related tools were chunky in section, with one face flatter than the other. The striking platforms of large flakes and the cortex of large cobbles were not necessarily removed entirely, resulting in asymmetrical handaxes. By the end of the Acheulean, very sophisticated handaxes were often made; flat and symmetrical in shape, they required great skill to produce. Elaborate core-preparation (e.g., Levallois) techniques, characteristic of Middle Paleolithic industries, were employed in producing highly refined bifaces in the latest Acheulean. Although many Late Acheulean assemblages exhibit refined skills in toolmaking, others are characterized by crude bifaces and bold flaking, typical of the Early Acheulean. Indeed, many factors affected the degree of sophistication of bifaces, including the raw material used. Overall change in the Acheulean is reflected by the fact that no Early Acheulean assemblage is known to be as refined as some Late Acheulean tool kits.
Lithic assemblages referred to as chopper-chopping tool industries are also known from the same time period throughout the Old World. These tool kits are typified by basic core-and-flake technology and tend to lack handaxes. Examples include the Clactonian in northern Europe, the Buda industry represented at Vértesszöllös (Hungary), and the Zhoukoudian industry in China. It is unknown whether these assemblages represent a distinct tradition of tool manufacture, geographic variants of the Acheulean, or, in some cases, an integral part of this industry. For example, it has been claimed that Clactonian assemblages reflect stages in the production of Acheulean tools. Other evidence suggests that biface and nonbiface assemblages are found in different habitats in the same area, as at Olorgesailie, and perhaps reflect different activities carried out by the same people. On the other hand, it is clear that assemblages in certain geographic regions, expecially in eastern Asia, simply are not characterized by bifaces.
At many Acheulean sites, bifaces occur in extremely dense concentrations in fluvial contexts. The behavioral interpretation of these sites is problematic due to the long time typically represented by fluvial strata and the possibility of winnowing of small flakes, leaving the heavier bifaces behind. While some Acheulean sites thus represent long periods of lag accumulation (similar to cobble bars in a stream), others appear to reflect the systematic deposition by hominids of handaxes near channels and of scraper-flake assem blages in floodplains away from the channel axis. The behavioral reasons for this pattern are unknown.
It is widely assumed that most Acheulean assemblages were manufactured by populations of Homo erectus. Fossils of H. erectus, however, are only rarely associated with Acheulean tools (e.g., at Tighenif [Algeria], Olduvai, and perhaps Swartkrans [South Africa]). In Africa, the oldest occurrences of the Acheulean (e.g., Konso and Olduvai middle Bed II) are in the time range of H. erectus (e.g., Olduvai Hominid 9). But
after 700Ka, they also occur at sites (e.g., Saldanha [South Africa], Ndutu [Tanzania], Bodo [Ethiopia]) yielding fossils often assigned to archaic Homo sapiens. In Europe, Acheulean assemblages first occur soon after 0.5Ma. Acheulean tools persist alongside early H. sapiens populations in Europe (e.g., at Swanscombe) and Africa until they are succeeded by Middle Paleolithic tool kits ca. 250–150Ka.
It is further assumed that these Acheulean toolmakers were hunter-gatherers who ranged widely for food. In fact, little is really known about the specific behavior and ecology of these hominids—for instance, whether they hunted big game or how they used their environments. Despite the prevalence of handaxes over an enormous time span, little is known about how they were used. One study of microscopic edge wear has shown that European handaxes were sometimes employed in butchery activities, and associated flakes also showed signs of working wood, hide, and bone. At other sites (e.g., an elephant skeleton and associated lithics excavated at Olorgesailie), handaxes evidently served as the cores for sharp flakes used in butchery. Experimental studies have indicated that bifaces are excellent all-purpose tools; their widespread distribution over much of the Paleolithic appears to bear this out.