Paleoclimate and Homo Migration out of Africa

Jacqueline Eng

Writer’s comment: Whenever a class requires a research paper, I always look for a novel topic, one that has been in the corner of my mind as something of interest but that I have never had the opportunity to study. Anthropology 5 (Proseminar in Biological Anthropology), and Professor Rodman’s encouragement, gave me the chance to study an aspect of hominid evolution that I had previously been too scared to approach: how abiotic factors, namely climate, may have acted as a selective force upon hominids, giving them the preadaptations necessary for their first migrations out of Africa into other continents. I had been scared because the arguments seemed so technical. But once I started to research, I enjoyed my topic more and more. After a quarter of work on this paper, I can proudly say that I have tackled a new topic and have learned something about human evolution in the process.
- Jacqueline Eng

Instructor’s comment: Anthropology 5 is titled “Proseminar in Human Biology.” It helps anthropology majors pursuing the B.S. degree to understand the relationship of our requirements in basic sciences to the study of humans. Jackie Eng enrolled in the class in Spring 1998. We began with introductory chapters from E.O. Wilson’s recent book, Consilience,in which he advocates integrating all paths to understanding, urging broad study of the sciences, social sciences, and humanities for specialists in all of these areas. Then we read and discussed a series of topics, from the origins of life and origins of thinking about the origins of life to the biological origins of morality. Biological anthropology demands consilience because it draws on all the natural sciences to interpret evidence of human origins and diversity today. Biological anthropology must also deal with uniquely human culture, including ethics and morality, in the context of biology—not in isolation from biology. The assignment for the term paper was to pursue in depth any subject of biological anthropology the student wished. Jackie Eng has written a thorough review of evidence addressing a critical point in the history of the genus Homo:the time of the first dispersal of hominids from Africa by the species we call Homo erectus.In succinct prose, she examines the climatic circumstances and physical adaptations that allowed or compelled some populations of Homo erectusto move out of Africa and throughout the Old World around 1.5 million years ago. Her review shows the integration of diverse sciences to discover and reconstruct this ancient prehistory.
- Peter Rodman, Anthropology

Introduction

    Africa is the birthplace of hominids, with the oldest hominid discovered yet, Ardipithecus ramidus,appearing nearly 4.5 million years ago (mya) in Africa (Wood, 1994). Hominids continued to develop within Africa for the next few million years. A new hominid, Homo erectus,which first appears in sub-Saharan Africa about 1.88–1.90 mya (Feibel et al., 1989), initiated the first migrations from Africa to other parts of the Old World. Before and during this time, the climate had been undergoing changes worldwide; in certain regions, increased aridity caused a shift in habitat and vegetation. As a possible consequence of this climatic process, H. erectusdeveloped a suite of physiological features which link it more closely with modern hominids than earlier hominids—in particular, lower limb lengths and a nasal morphology similar to those found in modern humans. There has been heated debate as to what led to this development of longer limbs, which preceded migration; were longer lower limbs an adaptation for locomotor efficiency, or were long legs a result of adaptations for more arid environments? This paper will explore how the Plio-Pleistocene climate, and not locomotor efficiency, may have been a selective force for the preadaptations in H. erectusmorphology necessary for migration out of Africa.

Climate and Evolution

    How abiotic factors such as climate relate to evolution has long been explored. Vrba (1995) suggests that the major evolutionary events punctuating the East African hominid fossil record were mediated by climatic conditions. There was a development of periodically cooler and drier African conditions after 2.8 mya, with subsequent intensifications at 1.7 and 1.0 mya (deMenocal, 1995). Dating techniques included the use of the oxygen isotope ratio and marine eolian records. After 2.8 mya, when ice sheets grew so that large glacial cycles could be sustained, the African climate became sensitive to small changes in high-latitude climate. Vegetation in East Africa (where H. erectusis first found) shifted from closed canopy to open savannah vegetation beginning in the mid-Pliocene. This marked a development toward reduced and seasonally contrasted rainfall. deMenocal and Bloemendal (1995) have found that East Africa showed a pronounced dry period between 1.8 and 1.6 mya. The Olduvai subchron (from approximately 1.9 to 1.84 mya) also appears to have resulted in significant paleoecological changes (Bar-Yosef, 1995). These climatic events coincide with a dramatic faunal turnover of suids between 2.0 and 1.6 mya, where there were three originations and three extinctions of species. The pattern of hominid origination and speciation during this same time is similar to that of suids (White, 1995).
     Events in other parts of the world corresponded to these climate changes. Han et al. (1997) have studied soil formation in loess-paleosol sequences, loess being a fine grained fertile loam deposited by wind and indicative of arid conditions. They have found that in central China during the period of 2.47–1.9 mya, there was a pronounced shift toward warmer and wetter conditions. The regional vegetation varies from forest-grasslands to sparse dry steppe, leading them to suggest that the climate fluctuated from sub-humid to semiarid/arid temperate zones. These climates were similar to those found in Africa, and we find that when H. erectusdoes migrate northward, it does not migrate any higher than 30° latitude, above which the weather is much more temperate. It therefore lived in a thermal range of 22°C with a minimum temperature of 11°C; likely, thermoregulatory stress from exposure to extreme winter seasons was a large barrier to the expansion of hominids adapted to more arid conditions (Foley, 1993).
     Thus the time when H. erectusdeveloped and migrated from Africa was marked with climatic events which produced more arid conditions. We now turn to look at H. erectusmorphology, and will later see how climate may have influenced it.

H. erectus Morphology

    Recently, there has been debate as to the legitimacy of the H. erectushypodigm because of the variation seen between the Far East assemblages and the African and Eurasian specimens (Wood, 1996; Rightmire, 1995). For the purposes of clarity in this paper, we follow the traditional view that the Far East, African, and Eurasian specimens attributed to H. erectusconstitute a single species.
     H. erectusis distinguished from its predecessor, H. habilis,by an increase in brain size (from 800 to 1100 cc), a reduction of the postcanine dentition, a vertical shortening of the face, a pronounced supraorbital torus, and the forward projection of the nasal aperture (Klein, 1989). H. erectusis also marked for its femur and pelvis, which are much like those found in modern humans. Differences from modern people include the robusticity of the pelvis and femur, the latter having pronounced muscle marks. It is also known for its development of long limbs, indicative of greater stature relative to that of earlier hominids.
     Walker (1993) has studied six H. erectusindividuals from East Africa and has obtained an average stature of 170 cm and an average weight of 58 kg. Similarly, McHenry (1991) has calculated female African H. erectusto be 160 cm and males to be 180 cm, for an average stature of 170 cm. These hominids were as tall as modern humans. Ruff and Walker (1993) have estimated the subadult male KNM-WT 15000, which is a nearly complete skeleton, to have been approximately 160 cm at death, with a projected adult height of 185 cm, and linear in body build. They also have found the body weight for African H. erectusto have been 63 kg for males and 52 kg for females. These estimates are comparable to body weight in modern populations—65 kg for males and 54 kg for females (Ruff and Walker, 1993)
     In nasal morphology, despite intraspecific variation, the total pattern is characterized by a projecting nasal skeleton, which is most marked in the superior nasal region, including the nasal bones, frontal processes of the maxillae, and superior parts of the lateral walls of the piriform aperture (Franciscus and Trinkaus, 1988). These skeletal features closely resemble those of modern H. sapiens.Similarly, H. erectusis the first hominid to have the modern human labyrinthine morphology (Spoor et al., 1994).
     H. erectushad many features resembling those of modern humans. Before we address what part climate may have played, we look at some of the sites where the earliest H. erectuswere found, to see when and where this species migrated.

Early Sites Outside Africa

    The oldest known Java hominids, Mojokerto 1 and S27 and S31, come from two localities (Anton, 1997). Using ⁴⁰ Ar/³⁹ Ar ages for volcanic units in direct association with the sites, in Perning and Sangrian, Swisher et al. (1994) determined the mean ages to be 1.81 ± 0.04 and 1.66 ± 0.04 my old. These dates are unexpectedly as old as some of the oldest African H. erectusspecimens, and this is not the only anomaly.
     In China, the site of Longgupo Cave near the eastern border of Sichuan Province has revealed early Homo,with a resemblance to East African early Pleistocene Homoand associated artifacts, which recall the Oldowan technology. Wanpo et al. (1995) used electron spin resonance analysis to get a date of 1.96–1.78 my for the hominid level site. The earliest H. erectusoccupation in temperate eastern Asia occurred when H. erectusmigrated from subtropical China across the Qinling Mountains to the Loess Plateau, where there is a site at Gongwanling, Lantian. Wang et al. (1997) used loess-paleosol stable isotope ratios to obtain a date of about 1.15 my for this site.
     In western Eurasia, the oldest hominid was found at the site of Dmanisi in East Georgia. The fossil, a mandible, shows similarities to both African and Chinese H. erectus.Gabunia and Vekua (1995) obtained a potassium-argon date of 1.8 ±0.1 mya. Another early Eurasian site is ‘Ubeidiya, Israel, dated to be about 1.4 my old (Belfer-Cohen and Goren-Inbar, 1994). In western Europe, some of the oldest hominids and pre-Acheulean artifacts come from the Gran Dolina cave site at Atapuerca, Spain (Carbonell et al., 1995). Pares and Perez-Gonzalez obtained a date older than 0.78 my for the site.
     These early dates of hominid presence outside of Africa before 1 mya have been disputed. Pope (1995) states that dating in the Far East has been hindered by the lack of secure provenience for the hominid finds and by lithostratigraphic inaccuracies. In particular, he says the published paleomagnetic stratigraphy of Java is completely at odds with the accepted geomagnetic polarity record. He also advises cautious acceptance of the dates obtained through the loess sequence in China. Roebroeks and van Kolfschoten (1994) argue the first solid traces of hominids in Europe are about 500,000 years old. Before this date, they say virtually all finds come from disturbed matrix, whereas after 500,000, there are primary context sites. Villa (1991) and Turner (1984) also suggest caution in accepting earlier dates.
     Until the debates over dating techniques are resolved, it is hard to determine when exactly H. erectusmigrated, but most investigators agree that a member of the Homospecies migrated from Africa. The earliest African fossil attributed to H. erectus, KNM-ER 2958, is an occipital fragment with affinities to the species (Leakey, 1976; Wood, 1991) dated to 1.88–1.90 mya. This date does not leave much time between H. erectus’ first appearance and the disputed early appearances of the species in areas outside of Africa, but new discoveries may reveal earlier dates in Africa. Roebroeks (1994) says the spread out of Africa was most likely eastward first, via ‘Ubeidiya and Dmanisi, into Asia, with Europe being occupied much later. Tattersall (1997) suggests that the very early migration of H. erectusfrom Africa explains the anomaly in the archaeological record: the stone tools found with the earliest African H. erectusare of the Oldowan technology, then at 1.4 mya the Acheulean industry arose in Africa but is not found in eastern Asia. He says the dates suggest that the first emigrants left Africa before the invention of the Acheulean technology. In any event, all these early finds in non-African continents, which come right after the attainment of a nearly modern body form, suggest that migration and morphology are connected.

Habitat Reconstructed

    Using the morphological adaptation of mammalian assemblages found with early hominids, Reed (1997) has reconstructed the habitat types found in Africa during the Plio-Pleistocene, including those in which H. erectuswere discovered. The KBS Member of the Koobi Fora Formation (1.88–1.6 mya) had dry, open conditions. The Natoo Member of the Nachukui Formation, West Turkana (1.6–1.3 mya) had wooded and edaphic grasslands, as well as swampy vegetation. Reed found that at 1.8 mya, grazing animals, indicative of more open woodlands and grasslands, increased to high percentages in East African regions. South Africa experienced a similar trend. A paleoenviromental reconstruction of the Nariokotome site where KNM-WT 15000 was found indicates a relatively dry, open environment (Feibel and Brown, 1993). The overall pattern in Africa indicates gradual aridification or change to more pronounced dry seasons.

Discussion

Long Lower Limbs and Locomotor Efficiency

    With the long limbs of H. erectus,many have wondered whether longer limbs result in a lower energetic cost of locomotion, and thus are an advantage for long distance travel and migration. Steudal and Beattie (1995) have found that limb lengths do not significantly influence an animal’s locomotor efficiency. Their results, however, do not rule out the possibility of a close relationship between cost and stride length.
     Holliday and Falsetti (1995) find that among modern hunter-gatherers, there is no evidence of a significant relationship between lower limb length and mobility, even when the effects of climate are held constant. Steudal (1994) notes that it is unlikely there is some undetected influence of limb length on energetic costs of locomotion since any effect would be a modest one that would be exceeded by variation in cost resulting from intraspecific variation in body size. She finds that the most important variable that affected locomotor efficiency and economy in early hominids was body size.
     Webb (1996) has tested two hypotheses: (1) that the interaction between the pull of gravity and an individual’s own upward acceleration determines at what speed one changes from walking to running, and (2) that increased lower limb length was selected for early hominids because of a locomotor advantage. He finds support for the gravity-based hypothesis (hypothesis 1): there is a positive correlation between maximum possible walking speed (Vmax) and lower limb length. However, he finds no selective advantage in Vmax in hominids since high walking speeds would be attained with greater efficiency by running. He concludes that the trend in longer lower limbs is unlikely to have been a result of selection for maximum possible walking speed. Further, maximum comfortable walking speed is not closely correlated with long lower limb length, so he concludes, like Steudal (1994), that increased lower limb length was not a result of selection for increased efficiency in walking.

Climatic Adaptations

    Many investigators now propose that thermoregulatory principles played a major role in H. erectus’adaptations in morphology. Bergmann’s and Allen’s rules, two “ecogeographical rules” (Mayr, 1956), define two aspects of the relationship between surface area and body mass. Bergmann’s rule states that within a polytypic species spanning a wide geographic range, larger-bodied variants live in colder parts and smaller-bodied variants in warmer areas. Allen’s rule states that under the same conditions, variants living in colder climates should have shorter limbs than those in warmer climates to reduce heat loss from the body core, while those in warmer climates should have longer limbs. Extremities are smaller than the body and are more drawn out and cylindrical, which increases surface area relative to mass (Ruff, 1994). A large surface area enables the loss of heat through increased radiation, convection, and evaporation, while a small surface area promotes heat retention.
     Wheeler (1993) has shown that a tall, linear body is advantageous for moving about in the open during the day. Relative to its mass, this body form experiences less heat gain from the sun, especially near mid-day, and greater convective heat loss from the body, especially during the morning and late afternoon. There is a reduction in daytime sweat losses, which in turn reduces total daytime drinking water requirements, which can then lead to increased day and home ranges. Wheeler (1991) has shown that a higher distribution of body surfaces also confers thermoregulatory advantages in a savannah environment because it allows the organism to deal with higher maximum heat loads due to an increased potential for evaporative and convective losses.
     Nearly complete skeletal specimens such as KNM-WT 15000 allow investigators to see what role climate has had on body morphology. H. erectusskeletons have limb proportions similar to those found in modern human populations, such as the Nilotic populations, who live in extremely hot, dry places (Ruff and Walker, 1993). Generalizing from Allen’s rule, Ruff and Walker (1993) predict that taller populations should be relatively more linear, that is, it should have smaller breadth/height ratio, than shorter populations living within the same type of climatic zone. They found this to be true; taller tropical populations are more linear because body breadth remains almost constant regardless of stature, and KNM-WT 15000 follows the same pattern. H. erectushad an increase in body mass over earlier hominids, without a large increase in body breadth, which made the body more linear so that it could still retain sufficient surface area to dissipate heat. Ruff (1991, 1993, 1994) and Ruff and Walker (1993) have suggested that since long limbs can be adaptations for more effective heat loss in hotter environments, then the long limbs of H. erectuswere adaptations to such environments.
     Isbell (1998) has suggested that the need to walk long distances to obtain food in warm, arid habitats with low productivity could have selected for long hind limbs and other adaptations in H. erectusfor more efficient thermoregulation. Ruff et al. (1993) find that there has been an exponentially increasing decline in diaphyseal robusticity within Homo that has continued from the early Pleistocene through living humans. They posit that their findings are consistent with a mechanical explanation, the declining mechanical loading of the postcrania, which suggests longer bouts of activity such as foraging.
     Leonard and Robertson (1997) have calculated the total energy expenditure (TEE) of several anthropoid species and human hunter-gatherers. The more active species consumed the more energetically rich diets. They find that TEE increased markedly in hominids beginning with H. erectus,which they say is partly due to larger body size as well as probable increases in day range and activity level.
     H. erectusalso shows adaptations for dry, arid habitats in its nasal morphology. For the lungs to function properly, the air in the terminal bronchioles and alveoli must be maintained at, or very close to, 100% relative humidity and body core temperature (Proetz, 1941; Negus 1958 as cited in Franciscus and Trinkaus, 1988). With a sample of 150 living populations, Weiner (1954) found a correlation of 0.82 between nasal index and absolute humidity. During expiration, air cools and the water vapor is recondensed on the walls of the passageway and in the cooler nose to retain body moisture. This moisture retention in mammals is enhanced by the external nose. Drier conditions are associated with more projecting noses. Unlike earlier hominids, H. erectushad fully anteriorly positioned external nose, probably with inferiorly directed nares (Franciscus and Trinkaus, 1988). This may be explained most parsimoniously in terms of increasing diurnal exploitation of resources in open country, particularly in arid regions.

Conclusion

    Hominid migration out of Africa to other parts of the world did not happen just one time in hominid history, but H. erectus’ migration marks the first migration. This paper has explored how H. erectusbecame the first hominid to journey beyond the African continent. In particular, the paleoclimate has been the focus as a possible selective force upon H. erectusmorphology so that it attained the preadaptations conducive for long-distance migration.
     This paper has looked at two possible selective forces for the preadaptation of the increase of H. erectuslower limb length comparable to modern human lengths. Longer lower limbs has been proposed to yield a lower energetic cost of locomotion, and may be thus advantageous for long distance travel and migration. Studies show that this is not likely, however, because there is no significant correlation between limb length and locomotor efficiency. The other selective force, climate, has more evidence supporting its role as a selective pressure.
     The time during which H. erectusevolved was marked by climatic changes, as can be seen with the increased aridification of the African continent, the country of origin for the species. This increase in temperature likely acted upon H. erectusas predicted by Bergmann’s and Allen’s rules. As seen in fossil evidence, the species increased in body mass, but without a major increase in body breadth so that the surface area/body mass ratio is kept low with this linear body plan. In addition, there was an increase of lower limb length to provide even more surface area. These morphological changes, a large body size and linear body shape, provided thermoregulatory advantages in the arid environment. There is also a change in nasal morphology for adaptations to the environment. All of these changes coincide with the aridification of Africa, and they may have facilitated prolonged diurnal activity and wide-ranging foraging in relatively open habitats. Movement out of Africa to other parts of the world with similar arid environments probably soon followed so that H. erectuspaved the way for future hominid migrations.

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