Photogallery of fossil skulls
This photogallery of fossil hominid skulls provides views of some of the key specimens and morphological changes discussed in the Handbook. Two other excellent sources for photos are Michael Day's (1986 edition) Guide to fossil man (University of Chicago Press) and Eric Delson's (1995) Ancestors: the hard evidence (Alan R. Liss, New York).
Hominid taxonomy is seemingly in a perpetual state of flux. Some notes alerting the reader to major classifactory changes are provided in the introductory chapter by Campbell, the editorial appendices to Holloway's chapter, and the epilogue chapter by Peters. Both the stability and the change characteristic of palaeoanthropology over the past few decades are testimony to the field's maturation.
An outline of human phylogeny
In 1871 Charles Darwin was able to propose that we were most probably of African origin and most closely related to the Great Apes of Africa. Biochemical evidence now reinforces this conclusion and indicates that the divergence of our lineage, the Hominidae, from the African apes took place between 5 and 8 million years ago (m.y.a.).
There are no fossils now believed to lie within our hominid lineage before c.6.0 m.y.a. The earliest group of well-known undoubted hominid fossils comes from Laetoli in Tanzania, and dates from c.3.7 m.y.a. These belong to the genus Australopithecus, which is considered to range in time from c.5 m.y.a. to 1 m.y.a., and appears to have been confined to the continent of Africa. Australopithecus was a bipedal, small-brained hominid, which later diversified into 2-3 more robustly built species, as well as probably giving rise to members of our own genus, Homo.
The earliest fossil remains classified as Homo, and thought to be our direct ancestors, come from south-west Ethiopia and adjacent Kenya. They are dated to c.2 m.y.a. This species, named Homo habilis, possessed a somewhat larger brain than Australopithecus, and appears at approximately the same time as the earliest stone tools. The successor to Homo habilis was the much more modern-looking Homo erectus. The earlier specimens are from Kenya, and date to c.1.5-1.8 m.y.a. Only after c.1.0 m.y.a. do we find that this species of Homo has spread into Eurasia. Archaic forms of Homo sapiens are variously recognized from Afro-Eurasian specimens dated to c.300 000 years ago. Recent biochemical data suggest that modern humans, Homo sapiens sapiens, arose in Africa c.200 000 years ago. This fits in well with the available fossil evidence from Africa and the Near East, where human skeletal material with completely modern features is known from an earlier date than elsewhere [eds].
Evolutionary trees of apes and humans from DNA Sequences
Peter J. Waddell and David Penny
Developments over the past decade have made DNA sequences the primary source of information for inferring relationships between organisms. Originally sequences were used for studying relationships between species, but increasingly they are now used to study relationships between individuals and between populations. In this chapter we show how sequences have changed, and continue to change, our views of human origins and evolution. Techniques used to go from DNA sequences to evolutionary inference are outlined, because they are crucial in evaluating this vast new source of data. In addition to a review we report some of the latest research findings, and where necessary have developed appropriate statistical methods. The main points of this chapter are:
1. There is consistently strong support for the human and chimpanzee lineages' being the closest relatives to each other, and the next closest the gorilla lineage, with the orang-utan being the closest non-African relative of these African hominoids.
2. A calibration of these evolutionary trees is given, with estimated dates of divergence for the living hominoids, together with estimates of the expected errors, an important consideration for those interested in assessing the compatibility or otherwise of fossil (or palaeoanthropological) data with molecular inferences. We estimate that the divergence of human and chimpanzee lineages took place approximately 6.5 million years ago, while the standard error of such dating methods is at present about 1 million years.
3. Our evaluation of the 'Out of Africa Hypotheses' (mitochondrial 'Eve') leads to the conclusion that this set of four hypotheses (pertaining to the when, where, who, and how of modern humans' origins) does indeed stand up to scrutiny; a point reinforced by our reanalysis of specific features of the data. No single data-set gives overwhelming support to all four aspects of the Out-of-Africa scenario; but it is consistent with several data-sets, while overall the data contradict the 'multiregion' hypothesis of human origins.
4. A re-evaluation of the molecular evidence confirms that the 'when' was almost certainly less than 200 000 years ago, as inferred from both mitochondrial and nuclear DNA data calibrated using both biological and palaeoanthropological data. Africa is most consistently inferred as the 'where'. The mitochondrial DNA sequences give us a glimpse of 'who' founded populations outside Africa and 'how', as populations appear to have expanded rapidly at some point after their arrival into new lands.
Evolution of the human brain
Direct palaeoneurological evidence about the evolution of the hominid brain comes from study of the size and surface features of the endocasts of once-living brains. Because of intervening tissues, the detailed surface features of the brain are seldom clearly expressed on the inside surface of the skull. Therefore convolutional details of the brain's surface are the least reliably preserved features. Cerebral asymmetries are more reliably preserved. Overall size, despite its questionable significance, is the most reliable evidence of evolutionary change.
In the last 3-4 million years brain volume within the hominid lineage has increased from less than 400 ml to roughly 1400 ml. The first clear increase in hominid brain size is seen in early Homo at c.2 m.y.a. in East Africa (most reliably in cranial specimen KNM-ER 1470). This is an evolutionarily significant change that cannot be simply accounted for in terms of increased body size alone. From the appearance of H. erectus at c.1.7 m.y.a. to the present, the brain increases nearly twofold: from c.800 ml to 1500 ml in Late Pleistocene H. sapiens, without any apparent change in body size.
With regard to brain reorganization, left-right cerebral hemispheric asymmetries exist in extant pongids and the australopithecines, but neither the pattern nor direction is as strongly developed as in modern or fossil Homo. KNM-ER 1470 shows a strong pattern that may be related to handedness and tool-use/manufacture. The degree of asymmetry appears to increase in later hominids.
The appearance of a more human-like third inferior frontal convolution provides another line of evidence about evolutionary reorganization of the brain. None of the australopithecine endocasts show this region preserved satisfactorily. There is a consensus among palaeoneurologists that the endocast of the specimen KNM-ER 1470 does show, however, a somewhat more complex and modern-human-like third inferior frontal convolution compared with those of pongids. This region contains Broca's area, which in humans is related to the motor control of speech. Unfortunately, later hominid endocasts, including H. habilis and H. erectus through archaic H. sapiens to the present, seldom show the sulcal and gyral patterns faithfully. Thus nothing palaeoneurological can be said with confidence about possible changes with the emergence of anatomically modern H. sapiens. On the other hand, there is nothing striking about Neanderthal brain casts in comparison to more recent H. sapiens, except their slightly larger size, suggesting no significant evolutionary change thereon [Eds].
Evolution of the hand and bipedality
Symbolic behaviour among humans and non-human primates incorporates the hands, and in human ancestors opportunities to use the hand for this purpose must have increased with the evolution of habitual bipedal posture and locomotion. In tracing the evolution of human symbolic behaviour it is therefore important to trace the origins of human bipedality, and to explore the progressive changes in hominid hand structure and functions that may have affected the use of the hands in communication.
A comparison of modern human hands with those of non-human primates reveals features unique to humans. Functional analyses of these unique features have shown that they are consistent with the stresses and requirements for joint movements associated with effective use of hand-held paleolithic stone tools. Hominid fossil hands from the Pliocene and Pleistocene provide some evidence of the sequence in which these features evolved. Structural adjustments to bipedal posture in the earliest hominids may have been an important correlate to developments in the hand, facilitating the use of the trunk as leverage in accelerating the hand during tool-use.
The evolutionary state of manipulatory potential of hominid hands has probably never been a limiting factor in gestural communication or in the manual creation of symbols.