the environmental background of hominid emergence and the appearance of the genushomo

HUMAN EVOLUTION Vol. 6 - N. 2 (129-142) - 1991

P.V. Tobias Director, Palaeo-anthropology Research Unit Department o[ Anatomy and Human Biology University of the Witwatersmnd, Johannesburg - South-A/rica

Key words: hominid emergence, genus Homo, environment and evolution.

The Environmental Background of Hominid Emergence and the Appearance of the Genus Homo

The human biologist usually considers ecology of recent humanity. This essay explores the question of whether the human biologist specialising in the ecology of living peoples has anything to learn from the palaeo-anthropologist, studying ancient hominids and their adaptive mechanisms over a deep time dimension. Since the Hominidae are under discussion, the definition of the hominids is reviewed. Historically, three phases are recognised. A rethinking of the classification of the hominoids has become necessary for the old and classical systematics, which divided this superfamlly into the Hominidae and the Pongidae, is now outmoded. Since no consensus on such a re-classification has yet been reached, the author adheres to the classical system for the time being. The Hominidae emerged between about 8 and 5 million years ago. At that time, Africa was subject to major cooling and aridification and considerable changes in the flora and fauna were occurring. Wet forests were retreating, savanna was spreading and the animals of Africa were undergoing many changes, partly by faunal interchange with Asia following the drying up of the Mediterranean, and partly by autochthonous evolution among the pre-existing species of the continent. The Hominidae could well have emerged from the striking environmental modifications of this late Miocene phase. Critical changes occurred in hominid evolution between 3 and 2 million years before the present. The pre-existing species Australopith- ecus a[ricanus acquired the form of a postulated derived A. africanus; the hominid lineage underwent cladogenetic splitting into robust and hyper-robust australopithecines and the genus Homo; Homo habilis appeared; stone tools are first found in the archaeological record; spoken language seems to have been acquired. These sensational events, within the space of one million years, took place against the background of conspicuous changes in the climate and physical geo- graphy of Africa, the flora and non-hominid fauna. Mankind became increasingly dependent upon stone culture. Hence a new element was added to the range of modes of adjustment, an element which must have greatly increased the ecological flexibility of the hominids. From the end of the Pliocene era onwards, culture should be seen as a constituent of man's environment and, at the same time, a highly advantageous component of human adaptational processes. In later and recent mankind, it may be difficult to extricate the respective roles of biological, social and physical factors, on the one hand, and cultural aspects on the other, as mechanisms and facilitators of adaptation to diverse econiches.

Introduction

I t is t he p r imary purpose of th is essay to ask - - and to t ry to suggest some answers to - - the ques t ion: Does the h u m a n biologis t , special is ing in t he s tudy of the ecology of l iv ing peoples , have any th ing to learn f r o m the pa laeo-an thropologis t , w h o devo tes h imse l f to the s tudy of changing aspects of anc ien t and ancest ra l h o m i n i d popula t ions?

�9 Editrice 11 Sedicesimo - Firenze ISSN 0393-9375

130 TOBIAS

It is a commonplace and readily observable fact of life that everchanging humanity adapted, and continues to adapt, to diverse environments. However, despite the enormous worldwide research effort channelled into the Human Adaptability Section of the International Biological Programme between 1962 and 1972 (MALHOTRa, 1966; YOSHI- MUgA & WF.INER, 1966; WANr,~, 1968; WEINER, 1969; COLLINS & WEINER, 1977), some questions remain unanswered.

We still need to know to what relative extents biological and cultural factors contribute towards adaptation in different populations living under varying environmental conditions. Of course, even biological factors alone are not a simple amalgam of traits. We must distinguish among morphological and morphometric features, monogenic and oligo- genie traits (such as those for proteins and blood groups), biochemical composition and reactivity, physiological activity and its limits of plasticity, psychozoic operations and behavioural flexibility. Thus, one way of looking at the problem is to try to determine the respective roles of morphological change, genie differentiation, functional response and cultural adaptability in the accommodating of the myriad human populations in their variegated eeoniches.

Logically, another kind of question would follow: Do different kinds of environment elicit different patterns or modalities of response in modern human beings? In other words, do some kinds of ecological challenge evoke predominantly cultural adaptation, with little or no morphometric or physiological change? Do others, again, elicit functional responses with few morphological or cultural adjustments? In other words, if we postulate the existence of a spectrum of possible patterns of adjustment among different living human populations, on what variety of econiche and of other factors depends the particular band of the spectrum operative in a specific population's adaptation to its present circumstances?

Alternatively, or additionally, to what extent does the mode of adaptation evident in a particular instance depend upon the group's contemporary environment, and to what degree upon the duration of the population's exposure to those conditions? This aspect of our probing brings us face to face with the time dimension and this, in turn, presents us with three or more sets of changes: demographic trends, secular tendencies and, finally, evolutionary transformations.

Demographic statistics on a population may play a part in conditioning its response to current and to changing conditions. At the same time, the nature of the demographic pyramid at any time is itself the result of changes in the immediate past and a pointer to ecological adjustments in the short term.

The secular changes to which recent human populations have been subject are a second category of short-term changes. It is well known that the intensity of positive secular changes in a population varies among different social strata (AcHESON & FOWLEg, 1964; BAKWIN & McLAUGHLIN, 1964; DAMON, 1968; TANNER, 1978; SUSANNE, 1984; PROKOVEC, 1984). Moreover, it has been shown that while 'First World' communities have in the recent past shown positive secular changes (e.g. in mean adult stature), 'Third World' communities have in numerous instances not manifested a positive secular trend; they have shown, rather, either no change (absent secular trend) or a reverse or negative secular trend (KENNTNER, 1963; VOGEL, 1971; TOBIAS, 1972, 1975a, 1975b, 1978a, 1978b, 1985a, 1986a, 1988, 1990; EVELETH et al., 1974; FRISANCHO et al., 1975; TOBIAS & NETSCHER, 1977; MALINA et aL, 1980; ~V]ALINA et al., 1983; BOGIN & MACVEAN, 1984; PRICE et al., 1987). For studies on the current status of humanity, we are confronted with the problem of how to make it possible for the majority of humankind to realise a greater proportion of its genetic potential than it is doing at present.

THE ENVIRONMENTAL BACKGROUND OF HOMINID EMERGENCE ETC. 131

In the context of the present discussion, the subject of secular changes (or the absence of them) has been brought in here because it would be of interest to examine the adaptive value of the varying intensity of secular change. The occurrence of secular changes of varying intensity and differing directions among world populations may reflect merely a seemingly passive response to favourable or disadvantaged living circumstances, or there may be an adaptational aspect to secular modifications. If further research proved the latter alternative to be correct, we should need to add the potential for secular changes, tO morphological, genetic, physiological and cultural factors, as yet another element in the repertoire of diverse adaptive strategies of humankind.

If we plumb the time-depth more deeply than that associated with demographic and secular trends, we reach, eventually, the realm of adaptation in evolution. We come up against another kind of question: if there are varying modes of adaptation among different living populations, and varying strata within those populations, was there similar diversity of adaptive mechanisms in the geological past? Was a particular band of the adaptational spectrum, or a limited range of bands, favoured among earlier hominids? Is there evidence, direct or inferential, of a change with time in the preferred mode of adjustment to the environment? Did the progressively increasing intensity of the cultural dimension in hominid life, revealed by the exposure of the palaeo-anthropological and archaeological record, bring in its wake changes in the relative importance of cultural devices in adaptation?

As a preliminary exploration of this interface between the living and the dead, the author presents an analysis of palaeo-ecological counterparts of a few of the key events in hominid evolution.

Climacteric events in hominid evolution

Palaeontological, archaeological and molecular lines of evidence lead to the recognition of at least fourteen major or climacteric happenings in the evolution of the Homini- dae from hominoid ancestors. These are conveyed in Table 1.

Of these fourteen seminal amendments, two lend themselves especially to palaeo- ecological analysis, namely the emergence of the Hominidae (no. 3 in Table 1) and the emergence of the genus Homo (no. 7). Both of these major events seem to have been accompanied by dramatic changes in the associated fauna, flora and climate.

The Hominidae

It is 233 years since the Swedish naturalist and systematist, Linnaeus, shocked his contemporaries by classifying man with the other animals. In giving humankind a place in nature, he at least made a separate genus of man in the 10th edition of Systema Naturae (1758), a work which is widely accepted as the starting point of modern classification and nomenclature (SIMVSON, 1945). He named the genus Homo and summarised its generic characteristics under the simple but effective definition, ~Nosce te ipsum>> (~Know yourselb>)!

It was not until 1825 that John Edward Gray proposed to place man in a separate primate family which he called Hominidae, a practice that has been followed by a majority of biologists and anthropologists to this day. At that time, the known members of the family comprised the living populations of the human species, Homo sapiens. The question of extinct members of the family had not entered the picture yet, because the discovery

132

TABLE 1 - Some climacteric events in hominid evolution *.

TOBIAS

Event Approximate dating

(millions of year before the present)

1. Divergence of orang-utah lineage from Hominoidea 17.3-14.0 2. Divergence of gorilla from other African hominoids 9.0-5.9 3. Divergence of chimpanzee from other African hominoids: emergence of

Hominidae 6.4-4.9 4. Earliest stone cultural remains 2.5- 5. Differentiation of derived Australopithecus a~ricanus 2.8-2.5 6. One or more splittings of hominid lineage 2.6-2.5 7. Emergence of Homo habilis 2.4-2.3 8. Acquisition of spoken language 2.5-2.0 9. Movement of hominids from Africa to Asia 2.0-1.5

10. Emergence of Homo erectus 1.7-1.6 11. Extinction of robust australopithecines c 1.2 12 Acquisition of control of fire 1.3 13. Emergence of Homo sapiens (problems of definition) 0.5 14. Emergence of modern human culture 0.1-0.025

* Modified after TOBmS, 1986b.

and recognition of fossil hominids started only somewhat later in the 19th century. Without this palaeontological dimension, it was possible to offer a zoological definition of the Hominidae (or hominids) which was based exclusively on the distinguishing features of modern man (as compared with those of modern apes).

Although most investigators continued to recognise humankind as a distinct primate family for the ensuing 160 years, the accumulation of fossil hominids of ever more ape-like men - - first Neandertal men in the middle of the 19th Century, then the much more primitive Pithecanthropus (now called Homo erectus) late in the 19th Century and the yet lowlier Australopithecus found by R.A. Dart at Taung in South Africa at the end of the first quarter of the 20th Century - have led to an erosion of the margin of distinctness separating men from apes.

Late in 1925 or early in 1926, very soon after the announcement of the discovery of Australopithecus, an American zoologist Harris Hawthorne Wilder prefaced his (undated) book on ~The Pedigree of the Human Race~ with these words:

~dn essaying to write of the Pedigree of the Human Race the author feels that he must write only as a professional zoologist, investigating the history of a single animal species which has become universally distributed and which, in the matter of the nervous system, has far surpassed the power of all other species>>.

Starting out from this premise, the author, perhaps not surprisingly, opines that ~there are certainly no structural differences sufficient to make a different Family of them (man past and present))~. Accordingly, trying to see the diversity of these higher primates as a hypothetical scientist visiting from the planet Mars might observe them, Wilder


makes the radical suggestion that the Family Hominidae should include, along with man, <<the tailless anthropoid apes and the various related species that have been recently unearthed in the soib~. As a compromise proposal, Wilder suggests that <<It may be useful to make a Sub-Family Homininae, to include man past and presenu~, comparable to the subfamilies proposed for the gibbons (Hylobatinae) and for the great apes (Pongiinae), but he adds, <<Even a definite separation of Sub-Families is hard to accomplish, and the distinctions between these may be eventually lessened when the returns are all in.~> (WILDER, 71926, page V).

GREGORY & HEta~MAN (1938, 1939), after their detailed studies of the teeth and jaws of the South African Australopitbecus, supported Wilder's view and proposed to add another subfamily, the Australopithecinae, to accommodate the various kinds of ape-man from Taung, Sterkfontein and Kromdraai. The colloquial form australopithecines is still widely used today.

However, the trend towards accommodating the apes in the same family as human beings suffered a major setback, if not a near-fatal blow, when in 1945 SIMPSON produced his masterful and authoritative classification of mammals. He placed all of the apes, including the gibbons, the extinct dryopithecines and, on his interpretation, the australopithecines, in one family, for which ELLIOT's (1913) name of Pongidae was accepted, while past and present members of Homo, as well as Pithecanthropus (now Homo erectus) and Eoanthropus (now exposed as the forgery of Piltdown), were assigned ~o their own family, the Hominidae. The enormous authority of Simpson's classification and the supporting enunciation of the principles of classification ensured that the apes and humans were kept apart in two different families ever since. This position was consolidat- ed with the publication of Le Gros Clark's ~The Fossil Evidence for Human Evolution~ (1955, 1964). He offered clear and extensive zoological definitions of the two families Hominidae and Pongidae. Despite the fact that early members of the two families were more alike than their later, derived descendants, including those living today, it was still possible to define the morphological trends characterising the two families in such a manner that even very early fossil primates could be seen to possess sufficient signs of the defining trends to enable them to be assigned to one or the other family.

Simpson had not overlooked the systematic affinities of the apes and man. To express their obviously close relationship, he had proposed to use another taxonomic device, namely the superfamily, in this case the Hominoidea (SIMPSON, 1931).

This then has represented the formal position of hominid and ape systematics for about the last half century:

Order: Primates Suborder: Anthropoidea Superfamily: Hominoidea Families: Hominidae, Pongidae

Wilder's assault on this system had been based on his appraisal of the morphology of these higher primates. More recent challenges to it have been fuelled by the startling results of biomolecular evolutionary studies. These newer data have flowed from pioneer- ing work on the molecular clock by ZUCKERr,~NDL & PAULmG (1962, 1965) and by SAlUCH & WILSON (1967). For it is a striking fact of hominoid evolutionary studies that the critical evidence bearing on the timing and the ordering of hominoid diversification, including the spawning of the hominids, has been furnished not by the fossil record (which is poor for the relevant time periods) but by molecular evolutionary data.

134 TObiAS

Molecular Reappraisals of Hominid-Ape Relationships

At one time, the anatomical study of the hominid fossils, enlivened by insights furnished by the comparative anatomy of related living creatures, provided virtually the sole basis of statements about hominid biological evolution.

Since the early 1960's, another group of scientists, the molecular biologists, have brought a new light to focus on the problem of hominid origins. The study of molecular systematics and evolution is predicated upon several main approaches: (a) the estimate of the 'immunological distance' between the albumins, collagens and other proteins of living species and populations; (b) the 'sequencing of amino acids', that is, the revelation by refined new biochemical techniques of the sequence of amino acids that make up a protein in the members of a modern-species: in a sense, a protein may be 'finger-printed' and comparisons may be made between corresponding proteins of different species, such as modern man (Homo sapiens) and chimpanzee (Pan troglodytes); (c) the sequencing of DNA (the repository of hereditary characteristics and the self-reproducing component of chro- mosomes and of many viruses), e.g. mitochondrial DNA sequences, and (d) DNA hybridi- zation studies when cells of different species are ~hybridize&~ together.

It is important to note that with one exception, these molecular studies involve inferences from the biochemical make-up of living organisms. The exception is LOWEN- STE~N'S (1980, 1985) radio-immuno-assay work on proteins, especially collagen, in fossil bones, provided the bones are not so old that all of the collagen has been replaced. In fossils over one million years old the amount of protein is extremely small (LOWENSTEIN, 1985). Therefore the radio-immuno-assay approach has not so far proved applicable to the fossilized bones of Australopithecus; nor has it thrown light on the order or the timing of the earliest branching of hominids away from ape lineages.

Molecular studies have furnished two kinds of information about the interrdation- ships of animals. One is the scale of proximity or distance between a number of related groups: thus, such data have shown that in biochemical make-up man is closer to the African apes than the latter are to the Asian ape, the orang-utan. Of the two African apes, in turn, a variety of data suggest a somewhat closer relationship between man and chimpanzee, than between man and gorilla. If, then, the molecules are to have a say in the determining of affinities, they point unerringly to Man, chimpanzee and gorilla forming one cluster of the hominoids and the orang-utan another division. This grouping means that, on molecular grounds, it would make better sense to recognise one family comprising past and present human beings together with African apes, and another family for the orang-utan and its predecessors (c.f. GOODMAN, 1962, 1963). Certainly, it has become extremely difficult to defend the grouping of the African apes and the orang-utan in a single family, Pongidae, as has been virtually universal practice for half a century.

Thus, the evidence of molecules supports a closer alignment of man, not with all the apes, but with the African great apes.

Recent molecular studies have therefore led to a revival of Wilder's 65-years-old proposal that the family Hominidae should include the tail-less great apes, at least the African ones. Wilder wanted to place all of the great apes in the hominid family; some recent investigators would place only the African great apes in the Hominidae; whilst an amalgam of chimpanzee and man, past and present, in a revised hominid family would gain some support. However, no consensus has yet been attained on this realignment of the higher hominoids; hence, in this article, the author will continue to use the term Hominidae for the family of man and his upright-walking ancestors, including earlier species of Homo and all species of Australopithecus.

T H E E N V I R O N M E N T A L BACKGROUND OF HOh~[INID E M E R G E N C E ETC. 135

Emergence of the Hominidae

We may regard the splitting of the African hominoids into chimpanzee and the residual hominoids as marking the origin of the Hominidae. On this thinking, the residual African hominoids - - i.e. after the splitting away from the African hominoids of, first, the line leading to the gorillas and, secondly, the line or lines of the chimpanzees - - constituted the Hominidae. The latest splitting event that generated the hominids has been variously dated (see review in TOBIAS, 1989), but we have here selected 6.4-4.9 millions of years before the present as embracing several estimates from molecular data which are, at the same time, not at variance with the scanty fossil data.

At this postulated time, the earliest appearance of the Hominidae is well correlated with appreciable climatic events. Some of the evidence from which climatic changes are inferred is based on the faunal assemblages associated with the hominids. It should be noted that not all faunal change connotes climatic change. For example, at the Miocene- Pliocene boundary, some five million years ago, the Mediterranean basin largely dried up. In this so-called Messinian crisis, a great dry route between Asia and Africa became available and a major faunal interchange occurred. During this period many palasiatic faunal elements appear to have been introduced to Africa for the first time. Another potential source of error in the interpretation of the fossil record is the accumulation of fossils under different environmental conditions. For example, BEHRENSMEYER (1976) has shown that the proportions of two genera of Suidae (pig family) represented in fossil assemblages varied according to whether the respective deposits had been formed under deltaic, channel or floodplain conditions. From such a taphonomic analysis, it may be inferred that a difference in the relative frequencies of two or more faunal elements might reflect differing local conditions (such as varying tectonic or geomorphological situations), or climatic variations, or both.

With due allowances for these alternative sources of variance, it is clear that in the latter part of the Miocene between 10 and 5 million years, when it is inferred that the family Hominidae arose, there were appreciable changes in Africa (HuNTLr~Y & WALKEg, 1982, Vm3a, 1985a). Over large areas forest and woodland were replaced by lighter, shrubland cover and savanna. It was a cooling and more arid Africa and BRAIN (1981) has suggested that the emergence of the hominids might even have been a consequence of this cooling event. VRBA (1985b) has suggested that the hominids were probably <4ounder members~> of the African savanna fauna! The tendency to cooling was heightened, or even occasioned, by a tectonic uplift of the continent, especially its eastern parts.

The living apes of Africa are to be found exclusively in the wet forest in the middle reaches of the continent. It is likely that ancestral anthropoid apes, too, were forest- dwelling creatures (cf. SnwoNs, 1985). The spread of lighter woodland and savanna and the retreat of the margins of the primaeval forests could well have created conditions in which the tendency to uprightness and bipedalism was favoured (TOBIAS, 1985b; SINCJ_~R et al., 1986). The ability to run across the high grass cover of the savanna, perhaps from one woodland-girt stream to another, might have held advantages for those apes which could ~walk talb>. Uprightness gave its possessors a chance to see over the tall grass and to watch out for predatory enemies like the lions and sabre-toothed big cats. Seemingly it was under just such a set of conditions that the Hominidae made their appearance upon the face of the earth.

It should be noted, however, that VV.RHAECEN (1987) has serious doubts that the hominid ancestors ever lived in the savanna; on physiological grounds he believes that ~Man is the opposite of a savanna inhabitant~>. He cites a number of features of modern

136 TOBIAS

mankind which distinguish it from savanna mammals: ~Humans lack sun-reflecting fur (MACFARLANE, 1968) but have thermo-insulative subcutaneous fat layers, which are never seen in savanna mammals. We have a water- and sodium-wasting cooling system of abundant sweat glands, totally unfit for a dry environment (MoNTAGNA, 1985). Our maximal urine concentration is much too low for a savanna-dwelling mammal (McFAR- LAND et al., 1979). We need much more water than other primates, and have to drink more often than savanna inhabitants, yet we cannot drink large quantities at a time (MACFAR- LAND, 1985; SCHMIDT-NIELSEN, 1979))>.

These are certainly telling arguments which need to be addressed. Verhaegen assumes that these biochemical and physiological traits characterised the emergent hominids of 7 to 5 million years ago, as they do to-day's mankind; we cannot be sure of this. Moreover the cultural and behavioural plasticity, including migratory propensity, of hominids might have been the hominids' evolutionary response to any built-in physiological constraints against savanna-dwelling. Thirdly, Verhaegen's claim that <<The fossils of our hominid ancestors or relatives are always found in water-rich environments)~ may be countered by two arguments. (1) We do not have a good supply - - or any - - fossil hominoids dating from the period when the molecular evidence points to the hominids' having emerged. (2) <~Well-watered areas)~ in Africa frequently mean wooded streams with feeder springs, successive streams being separated by stretches of savanna: in such an econiche, the emerging hominids could well have had the best of both worlds - - the foraging and scavenging advantages of life on the savanna and the physiological defences afforded by the proximity of the wooded streams and lakes. Lastly, we should not overlook the fact that the hominids emerged at the very time when drier conditions were causing the retreat of the middle African forest margins and the spread of the savanna. The powerful selective advantages of bipedal life, in or near the edge of the savanna stretches, strongly suggest that this synchrony in timing was no mere coincidence or serendipitous circumstance, but part of a pattern of causally interrelated events.

It was an era of dramatic change among the fauna too. A primitive member of the ursid or bear family, Agriotherium, made its last appearance, prior to its vanishing from Africa. New tribes of bovids showed themselves, including several endemic tribes of typical savanna antelopes (VRBA, 1985a, 1985b). Members of the elephant family appeared in Africa, as did hyaenas and sabre- toothed cats. The Suidae (pig family) diversified greatly and the rodent Otomys stood revealed. It is likely that some of these changes, especially those of some antelopes, elephants, carnivores and rodents, flowed from the major faunal interchange accompanying the <<Messinian crisis)~. That is, species new to Africa moved in to the continent from western Asia. Other changes, particularly of some of the bovid tribes (VRBA, 1985b), seem to have occurred among groups already present in Africa and to have been related especially to the late Miocene cooling and drying. Thus, the African savanna fauna made its debut about that time. Among them, it seems, walked the earliest hominids.

In other words, the Primates were not changing in a vacuum. AU Africa seethed with climatic shifts and biotic transitions and innovations; the eastern parts of the continent were cooler and more elevated; animals new to Africa were migrating into the continent from the old Asian ('Palasiatic') fauna.

In short the challenge for the emergence of the Hominidae could well have emanated from the striking environmental modifications of the late Miocene period. These circumstances should not be seen as confined to cooling and its direct effects on the group with which we are primarily concerned, namely the last common ancestors of the hominids and the African great apes. Rather, we have to deal with a much more complex skeinwork of


causal mechnisms. Behind the cooling were global trends (BoAz & BuRcr, J.~., 1984), Antarctic events (FLOHN, 1984) and intra-African upheavals in the form of a somewhat earlier, major continental uplift (PARTRIDGE, 1985). Each of these, in turn, affected Africa secondarily in diverse ways, such as through faunal changes, autochthonous and by immigration; through geomorphic responses such as the modification of drainage systems and the creation and demise of lakes; through deforestation and the spread of more open, less protective floral conditions; and through other forms of ecological diversification. With new immigrant animals, freshly-evolved endemic (or autochtonic) species, spreading savanna, and prevailing cooler, more arid conditions, environmental challenges to survival were more exacting.

From this African late Miocene maelstrom, one of the continent's small-brained, little specialised ape populations came forth as tottering bipeds with emancipated hands and fingers: they were the founding fathers and mothers of the family Hominidae, genus Australopithecus. Audaciously, they slipped the stagnating shackles and cosseting constraints of their former forest retreat. With curiosity, enterprise and opportunism, they ventured on to the plains of the high, inland plateau of Afr in time they conquered the savanna.

Critical Changes from 3 to 2 Million Years Before the Present

Several millions of years after the emergence of the hominids, we pick up the story again with the concluding million years of the Pliocene Epoch - - from 3 to 2 million years ago. In the hominid story, it is one of the most momentous periods in the record. No fewer than five of the 14 items listed in Table 1 are shown as having occurred, in all probability, within that span of time, namely the appearance of the earliest deliberately fashioned stone tools in the archaeological record; the acquisition of spoken language; the development of early Australopitbecus africanus into an advanced or derived A. a[ricanus; a critical and revolutionary moment in the hominid story, namely the cladogenesis or splitting of the lineage (at least once, but possibly twice) to generate the late surviving robust and hyper-robust australopithecines, known as A. robustus and A. boisei respective- ly (called by some investigators members of another genus Parantbropus), and the emergence of a new generic line, that of Homo, the earliest recognised species of which is Homo babilis.

This sensational series of evolutionary phenomena paved the way for the establish- ment of the lineage leading directly to modern humankind, Homo sapiens. It behoves us to examine the records carefully to determine what other phenomena occurred at this time, the last million years of the Pliocene Epoch.

While Australopitbecus a[ricanus, as we know it from Makapansgat and, especially, from Sterkfontein Member 4, was probably evolving the derived condition for a number of characters (as postulated by SKELTON et al., 1986), leading to the dramatic splitting of the lineage and the generating of the earliest members of the genus Homo, great faunal and floral changes were taking place.

The identified vegetation testifies to the supervention of cooler and more arid conditions (c.f. SHACICLETON & KENNETT, 1975), especially towards the latter part of the million year period (PARTRIDGE et al., 1990). These brought in their wake a second phase of the opening of the woodland and the increase of the savanna. Amid these drier conditions, a number of major biotic events occurred, apart from the emergence of Homo.

The present-day genus of baboon, Papio, appeared in the fossil record for the first

13 8 TOBIAS

time. A number of members of the family Elephantidae disappeared, as did certain suids and many archaic bovids; replacing them we find new, later suids and many modern species of bovids. The hexaprotodont hippopotami (hippos with six front teeth) gave way to tetraprotodont hippopotami (with four front teeth). The Machairodontinae (sabre- toothed cats) departed from the fossil record, as did several kinds of rodents, while some new rodents made their debut.

All of this evidence bespeaks other conspicuous changes in the flora and non-hominid fauna of Africa, when the genus Homo first strode forward in the terminal aeons of the Pliocene Epoch. Faunal interchange between Asia and Africa is less in evidence at this time than at 7-5 million years before the present, when the Messinian 'crisis' had facilitated major faunal movements between these two continents. Hence, the palaeontological evidence for all of these great faunal changes in Africa leads one to infer that vigorous biological evolution characterised the African continent in the final million years of the Pliocene. Man, in the sense of Homo, was born as part of this wave of change, to a world of drier conditions, a rather rapidly changing biota and a generally more challenging and exacting environment.

We see thus that our genus emerged under increasingly difficult and more demanding conditions of life in Africa.

Culture as part of human ecology at the critical cladogenesis

Human stone-working cultures seem to appear at the same late Pliocene period. Although for many years a close correlation has been deemed to exist between the earliest stone tools and the first appearance of Homo babilis, still older stone tools, the most ancient of which are now dated to c2.5 million years ago, have been brought to light at kada Gona and West Gona in Ethiopia (CoRvINUS, 1976; CORVINUS & ROCHE, 1976; T~aEB & TIERCELIN, 1979; ROCHE & TIERCELIN, 1977, 1980; ARONSON etal., 1977; TAIEB et al., 1978; WALTER, 1980; SCHMITT et al., 1980; WALTER ~x~ ARONSON, 1982; HARRIS, 1983).

These earliest known stone tools are older than the earliest well-attested occurrence of H. habilis remains by about 0.5 - 0.6 million years. Although this margin of difference is admittedly rather small when the limits of error of the dating determinations are borne in mind, yet the available dates, from the well-dated sequences of especially Omo and Gona West, suggest that stone tool-making first appeared prior to (and not coincident with, or subsequent to) the branching of the hominid lineage that generated inter alia Homo habilis. Along this line of thinking, the development of stone tool-making would have occurred among some of the populations of the derived or advanced A. africanus. This cultural advance could then have been a behavioural counterpart of the morphological features characterising the derived A. aMcanus.

On this evolutionary scenario, stone cultural activities would have provided a critical element in the environment of the derived A. africanus. The emergence of stone culture should thus be seen as part of the nexus of catalytic and causal events that led up to the splitting of the lineage. The tool-making habit would, after the split, have persisted as the potential preserve of one or both or all of the daughter lineages. While we cannot deny lithicultural proclivities to the robust and hyper-robust australopithecines after the split, it is the Homo lineage which seems to have made stone culture peculiarly its own. Homo habilis, it seems, became obligate stone tool-makers and users, whereas the robust australopithecines might have been only [acultative tool-makers and users.


Seen in this new light, culture - - as reflected initially by evidence of stone tool- making - - played an important, probably a crucial role in the genesis of Homo and of its earliest species H. babilis.

From that point onwards, the evidence indicates that the cultural dimension came to loom very large in the pattern of hominid life. The proliferation and diversification of the cultural component, with an inevitable deepening and widening of its cognitive underpin- ning, must have effected a considerable change in human adaptation. Whereas prior to the emergence of human cultural behaviour, hominid ecological adjustments must have been essentially biological and social in nature, thereafter the acquisition of a cultural dimension added appreciably to the range of possible mechanisms of adaptation available to the evolving human organisms.

Thenceforward, biological and cultural events were concomitants in humanity's further evolution. Indeed, it is often difficult to extricate the effects of one from the other. This interdigitation of biological and cultural mechanisms in the patterns of human adaptation has persisted to the present day.

We may thus envisage a spectrum of modes of adaptation. According to this, the survival of the earliest hominids seems to have been predicated essentially upon biological factors, such as morphology, physiology, biochemistry and social behaviour; whereas the survival and adaptation of at least some modern human populations is conditioned largely by cultural determinants. An example of such a present-day population is the San (formerly called Bushmen), in whose present-day ecological adjustments cultural practices play a predominant role (TOBIAS, 1957, 1960, 1984).

When human ecology is viewed against this deep time dimension, it would not be rash to suggest that the major change in the mode of human adaptation came about when cultural factors were added to physical, biotic and social components, as means of adaptation. The addition of cultural modes of adjustment must have greatly enhanced human adaptability. As cultural modes themselves became increasingly flexible, so their part in increasing the plasticity of human adaptive responses must have improved.

It is most likely that, from the late-Pliocene onwards, culture was at one and the same time an unavoidable and virtually universal part of the ecology of humankind and an inexorable component of human adaptational processes.

ACKNOWLEDGEMENTS - - I express my gratitude to the International Union of Anthropological and Ethnological Sciences and to Professor Napoleon Wolanski for his role in creating and leading its Commission of Human Ecology. Mrs. Heather White carefully and thoughtfuUy prepared the typescript.

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Received: 24 November 1990. Accepted: 12 January 1991.

the environmental background of hominid emergence and the appearance of the genushomo

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