Tuesday, July 8, 2014

Evolution of early Homo: An integrated biological perspective

Susan C. Antón, Richard Potts, Leslie C. Aiello
Science 4 July 2014:
Vol. 345 no. 6192
DOI: 10.1126/science.1236828


Integration of evidence over the past decade has revised understandings about the major adaptations underlying the origin and early evolution of the genus Homo. Many features associated with Homo sapiens, including our large linear bodies, elongated hind limbs, large energy-expensive brains, reduced sexual dimorphism, increased carnivory, and unique life history traits, were once thought to have evolved near the origin of the genus in response to heightened aridity and open habitats in Africa. However, recent analyses of fossil, archaeological, and environmental data indicate that such traits did not arise as a single package. Instead, some arose substantially earlier and some later than previously thought. From ~2.5 to 1.5 million years ago, three lineages of early Homo evolved in a context of habitat instability and fragmentation on seasonal, intergenerational, and evolutionary time scales. These contexts gave a selective advantage to traits, such as dietary flexibility and larger body size, that facilitated survival in shifting environments.

Introduction [quoting from the paper]

The evolution of the genus Homo has long been linked to the onset of African aridity, and the evolution of key features such as increased carnivory, brain enlargement, long-distance mobility, and prolonged life history. These features have been explained as a response to the progressive expansion of open, grassland habitats (1, 2). However, new environmental data challenge this interpretation, and archaeological research has identified behaviors in early toolmakers that aided flexible responses to dynamic environments (3, 4). Furthermore, comparative studies of mammalian development, energetics, ecology, and behavior offer new interpretive models. In this context, new fossils have also expanded the known range of morphological variation, raising questions about the number of species of early Homo and the distinction between inter- and intraspecific adaptations (5, 6, 7, 8, 9, 10).
The East African fossil record continues to command much attention because of a unique combination of factors. The history of East African rift volcanism enables precise geochronological analyses through long stratigraphic sequences rich in fossil and archaeological remains. The temporal sequence of morphological and behavioral innovations in early Homo is thus more finely resolved in East Africa than elsewhere. Environmental indicators can also be measured in lengthy stratigraphic order, enabling researchers to assess climate and habitat dynamics at a variety of time scales rather than relying on more limited environmental snapshots or broadly time-averaged portraits of the environment. Uncertainties over stratigraphic correlation and dating have arisen that directly affect an understanding of early Homo, yet East African rift basins typically offer opportunities to resolve the geological debates [e.g., (11, 12)]. Beyond this region, important recent finds pertinent to the evolution of Homo have been made at Malapa, South Africa (6, 7, 9, 13), and Dmanisi, Georgia (8), which expand how hominin morphological variation and the dispersal of early Homo beyond Africa are understood. This review begins with a focus on morphological variation and environmental dynamics because these topics have strongly affected analyses of the adaptive shifts distinctive to early Homo (Fig. 1). [below]

Fig. 1   Hominin evolution, diet, landscape vegetation, and climate dynamics from 3.0 to 1.5 Ma.
(A) Currently known species temporal ranges for Pa, Paranthropus aethiopicus; Pb, P. boisei; Pr, P. robustus; Aafr, Australopithecus africanus; Ag, A. garhi; As, A. sediba; H sp., early Homo > 2.1 Ma; 1470 and 1813 groups, see text for definitions (traditionally classified as H. rudolfensis and H. habilis, respectively); and He, H. erectus. The temporal position of Dmanisi H. erectus, He (D), is indicated. (B) Icons representing the first appearance of (from bottom) Oldowan technology (~2.6 Ma), Homo dispersal to Eurasia (~1.85 Ma), and Acheulean technology (~1.76 Ma). Horizontal pale green lines mark these times across (A) to (D). (C) Homo tooth δ13C. Carbon isotopic values measured on tooth enamel of East African specimens assigned to Homo and P. boisei (21); the mean and range of dental δ13C for A. africanus is also shown (22). (D) East African paleosol δ13C: compilation of δ13C values for East African fossil soil carbonates [data compiled in (74)]. Values range from those typical of woodland (40 to 80% woody cover) to wooded grassland (10 to 40% woody cover) to grassland (0 to 10% woody cover). Woody cover estimates based on (2). (E) Climate variability. Alternating intervals of high (lighter color bands) and low (darker color bands) climate variability based on predicted insolation resulting from the modulation of orbital precession by eccentricity, where low variability is defined by eccentricity ε ≤ 0.0145, (i.e., 1 SD below mean ε for the past 5 million years) (67). White circles show the standard deviations for terrigenous dust flux values at Ocean Drilling Project sites 721 and 722, western Arabian Sea (64, 69). Change between eolian dust standard deviations (adjacent white circles) follows the predicted direction between alternating high (larger SD, further to the right) and low (smaller SD, further to the left) climate variability for 13 of the 16 variability transitions. For example, the large SD in the two predicted high climate variability intervals, 2.79 to 2.47 and 2.37 to 2.08 Ma, is further to the right of the plot than is the intervening small SD in the predicted low-variability interval 2.47 to 2.37 Ma.

[ Body of the paper (blog comments):

The body of the paper includes sections "Who was early Homo?", "Environmental instability as an evolutionary paradigm", and "The paleobiology of early Homo" . These sections are important toward understanding the conclusions of the paper.  However, as the paper is currently not open access, I have not included them here.

There are also two important "box" discussions in the body of the paper:

Box 1: Anatomical features of early Homo groups (which details the methods used to classify the 1470 and 1813 groups, and early H. erectus discussed in Figure 1), and

Box 2:  Sympathry and niche partitioning in early Homo.

Key points from the box discussions:

Box 1: Anatomical features of early Homo groups: "Key members of Early H. erectus are from Africa and Georgia: crania and calvaria KNM-ER 3733, KNM-ER 3883, and KNM-ER 42700, OH 9, and Dmanisi 2280; crania and associated mandibles Dmanisi 2282/211, 2700/2735, and 3444/3900; crania and associated postcrania KNM-ER 1808 and 15000."

Box 2: Sympathry and niche partitioning in early Homo:  "carbon isotopic values for teeth of broadly sympatric representatives of early Homo in the Turkana Basin, Kenya (~1.99 to 1.46 Ma), call into question the idea that tool use precludes niche differentiation."

] (end of blog comments)

Conclusion: New frameworks and unresolved questions
[quoting from the paper]

A suite of morphological and behavioral traits once considered to define the origin of the genus Homo or of earliest H. erectus evolved not as an integrated package but over a prolonged time frame that encompassed species of Australopithecus, early Homo, H. erectus, and later Homo. The idea of an integrated package of traits in early Homo has been thought to anticipate the adaptive characteristics of H. sapiens and to include reduced face and teeth, a substantial increase in brain size, body proportions characterized by an elongated hind limb and shortened forelimb, essentially modern hand functional morphology, dependence on toolmaking and culture with incipient language capabilities, dietary expansion, persistent carnivory and systematic hunting, narrow hips with implications for the birth of altricial young, prolonged life history compared with extant apes, and cooperative food-sharing focused at a home base (15, 122, 123, 124, 125). New fossil and archaeological data summarized here allow refined perspectives on the morphological variation and pacing of evolutionary change in the Homo clade. These empirical findings, coupled with interpretive models drawn from developmental and comparative biology and behavioral ecology, now require the disentangling of this package of traits (Fig. 3). [below]

Fig. 3. Evolutionary timeline of important anatomical, behavioral, and life history characteristics that were once thought to be associated with the origin of the genus Homo or earliest H. erectus.

An important, continuing goal is to develop a more refined understanding of exactly what adaptive features did originate with early Homo. According to present data, facial and dental reduction defines the earliest members of the genus between 2.4 and 2.0 Ma. Cranial capacity expanded by 2.0 Ma. A greater yet varied degree of brain enlargement correlated with body size increase is expressed in early H. erectus between 1.9 and 1.5 Ma, although estimates of the degree of encephalization overlap with those of Australopithecus. However, brain expansion independent of body size appears to be most strongly expressed later, between 800 and 200 thousand years ago. A relatively elongated hind limb is present in A. afarensis (by 3.9 Ma) and in later Australopithecus (A. africanus, A. garhi, and A. sediba) but not in Ardipithecus (4.4 Ma). Absolutely longer and strongly built femora evolved between 1.9 and 1.5 Ma, coinciding with early H. erectus. Stone technology at ~2.6 Ma may predate the origin of Homo, whereas cultural capabilities of the early Pleistocene led to highly persistent traditions of toolmaking rather than an innovative, cumulative culture linked to symbolic behavior typical of the latter part of the Pleistocene. Transversely oriented hips and a broad pelvis persisted until H. sapiens, although a brain consistently >700 cm3, which occurred after ~1.8 Ma, connotes altricial neonates and heightened cooperation among H. erectus adults. Based on first molar dental histology and eruption, the tempo of life history was slower in H. erectus than in Australopithecus yet was similar to that of extant great apes. Far more prolonged phasing of growth typical of H. sapiens, with implications for intensive social cooperation, is evident in the middle Pleistocene, which is also when definitive evidence of hearths and shelters occurs in the archaeological record, implying strong centrally located social cooperation. The traits associated with the origin of Homo and of H. erectus thus evidently did not approximate the integrated complex of adaptations found in H. sapiens.

The evolution of early Homo, moreover, was associated with recurrent periods of intensified moist-dry variability (Fig. 1E). Dynamic environments favored evolutionary experimentation and the coupling and uncoupling of biological variables (71, 126), which governed against any simple transition from Australopithecus to Homo. We maintain that the East African record to date preserves three distinct taxa of early Homo, including H. erectus, although the issues that arise from recent discoveries elsewhere at Malapa and Dmanisi hint at the intriguing shuffling of derived and plesiomorphic traits and biological variables that likely characterized the early evolution of Homo.

Developmental plasticity and ecological versatility were at a premium in the habitats in which early Homo evolved. Although plasticity across biological levels (molecular to behavioral) was favored in dynamic habitats, both extrinsic (e.g., environmental) factors as well as biological and social feedback mechanisms were complexly entwined in the evolution of Homo and can no longer stand as alternative explanatory hypotheses (4, 61). Understanding the processes by which adaptability evolved in Homo and exactly how various traits contributed to plasticity during the evolution of the genus are important future challenges.

Critical foci for future research on the paleobiology of early Homo are numerous. To cite four examples, first, the field is always well served by new fossil and archaeological finds. Larger fossil samples between 2.5 and 1.5 Ma will be necessary to assess the taxonomic diversity of early Homo and to determine the temporal and spatial integrity of the morphological groups. Second, comparative mammalian studies focused on population structure, genetic isolation, niche differentiation, and the variables enabling the coexistence of congeneric taxa will help build more effective models for understanding morphological groups and diversity in early Homo. Third, much remains to be learned about encephalization in early Homo, the degree of plasticity in body and brain size, and how these variables were related to paleoenvironmental variables (e.g., shifting resource abundance). Last, interpretations concerning early Homo rely on the comparative biology of a wide range of mammals (including humans) in order to test and develop robust models of the intricate relationships between energetics, life history, brain and body size, diet, mortality, and resource variability across temporal and spatial scales. A refined understanding of these relationships will enable the union of many disciplines to yield a deeper understanding of human evolution.

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