November-December 1996
November-December 1996
Roy Larick and Russell L. Ciochon
Time, Climate and Species
The most basic questions for human dispersal have remained hypothetical
during the past two decades. When did hominids first leave Africa? Which
species was the first to leave? Why did they leave? The issue of age has
always overshadowed all others. In the eastern Rift Valley sites, fossils
are usually recovered from relatively fine-grained deposits laid down by
water and wind. These formations often include layers of volcanic ash that
are easily dated using the potassium-argon (K-Ar) radiometric method. Alternatively,
in Europe and subtropical Asia many fossils are found within the diverse
and complex deposits that accumulate in caves, where depositional histories
are difficult to interpret and volcanic materials are not present. The net
effect is that the age of Homo fossils has been measured more precisely
(and consistently older) in Africa than in Eurasia. Thus, the earliest Homo
erectus (better termed Homo ergaster) fossils in the eastern
Rift Valley appear fully developed by 1.9 mya, whereas the Javanese fossils
(classic Asian Homo erectus), which are thought to be the
earliest in Asia, have traditionally received broad age estimates of only
700,000 years to 1.1 million years. The nearly one-million-year disparity
between the African emergence and the initial Asian arrival has for years
been the basis of the conventional theory for a late dispersal.
Recent developments in techniques that provide the absolute ages of artifacts--such
as paleomagnetism, electron-spin resonance (ESR), and single-crystal argon
(Ar/Ar) methods--have shed new light on the arrival of Homo in Asia.
Moreover, the discovery of new artifact-bearing sites makes the dispersal
of Homo a much more accessible question. At Riwat and Pabbi Hills
in Pakistan, simple stone tools have a paleomagnetic age of about 1.9 million
years. At Sangiran and Mojokerto in Java (Indonesia) the sedimentary contexts
for three well-known cranial specimens of Homo erectus now have Ar/Ar
age determinations of 1.6 to 1.8 million years. The most intriguing of the
new finds for early Asian dispersal come from Longgupo, a cave in southeastern
Sichuan Province, China. The Longgupo hominid teeth have affinities to early
African Homo and the stone artifacts resemble early African tools.
Last year, we and our Chinese colleagues, Huang Wanpo and Gu Yumin at the
Institute for Vertebrate Paleontology and Paleoanthropology, published ESR
and paleomagnetic analyses that indicated an age of 1.9 mya for the Longgupo
remains. The growing number of Asian hominid fossils and stone-tool assemblages
that approach 2.0 million years in age now suggests that an early population
of Homo arrived in eastern Asia within a few hundred thousand years
of arising in Africa. In the light of this new evidence for early dispersals,
it appears Homo emerged not so much in adaptation to challenging
conditions, but fully poised to dominate new resources in new territories.
Crucial aspects of hominid evolution and dispersal evidently
relate to global climatic trends, particularly to the cooling and drying
associated with glaciation in the northern hemisphere. Within the Rift Valley,
such oscillations repeatedly added open components to woodlands, to effect
more mixed or mosaic-like habitats. By implication, climatic trends prompted
the large hominoids to develop new physical and behavioral adaptations,
but the process and results are not always clearly related. For example,
a cooling event during the Late Miocene epoch (about 6.0-5.3 mya) correlates
well with the time at which apes and hominids diverged as estimated by molecular
clocks. Although climate must have played a role in this speciation event,
fossils from this period have yet to be found. Consequently, the crucial
anatomical effects of climatic cooling and their relation to the divergence
of the apes and the hominids are not known.
Fortunately, it is easier to link the emergence and dispersal of Homo
with cooling during the Middle Pliocene (3.0-2.4 mya). During this period
in Africa, many mammalian species were pushed toward extinction, speciation
or dispersion. Yale University paleontologist Elisabeth Vrba has documented
extinctions for larger species of forest-adapted African bovids, particularly
the antelopes, and the emergence of more cursorial, open-dwelling species
that occupy grassland habitats to this day. Vrba also has shown that some
six species of African bovids, a relatively large number for any period,
dispersed to Eurasia during the Middle Pliocene. The dispersal of large
bovids is especially interesting because their shift from the forest to
the open resembles that hypothesized for the hominids. Moreover, since open-country
bovids could have been hunted or scavenged by Homo, the two groups
may have emerged and dispersed together.
Regarding the hominids, Middle Pliocene cooling underlies
the divergence of Australopithecus into two evolutionary lines. One
yielded Paranthropus, whose robust jaws and massive teeth reflect
a rather specialized coarse vegetarian diet. In this robust line, Paranthropus
boisei survived until 1.2 mya and may have developed stone tools. Nevertheless,
in other ways Paranthropus was not substantially different from its
australopith ancestors. With sexually dimorphic bodies and ape-sized brains,
Paranthropus was confined to tropical habitats and never encountered
its continental limits.
The second, more omnivorous line issued Homo in the African fossil
record. From the discovery of Javanese finds in 1891 to the early 1960s,
when Louis and Mary Leakey's work began to pay off in spectacular finds
at Olduvai Gorge, our own genus comprised just two species: Homo erectus
(identified only later in Africa and, by some accounts, in Europe) and our
own Homo sapiens. With the Leakeys' discoveries of Homo habilis
at Olduvai, Homo erectus's direct antecedent seemed to have appeared
at 1.8 to 1.6 mya. When first found, Homo habilis presented a larger
cranium and narrower teeth than the older autralopiths and yet still showed
some primitive features such as small size. As fossil finds of Homo habilis
have accumulated more recently, apelike long arms, short legs and australopith-like
thigh anatomy have cast doubt on this relationship. Homo habilis is
morphologically too primitive to be an ancestor of Homo erectus.
Concurrently, a number of fairly complete and well-preserved
fossils from the Turkana Basin of northern Kenya (as well as a skull from
Swartkrans, South Africa) indicate that Asian Homo erectus has an
equally ancient sister species in Africa. By 1.9 mya Homo ergaster
exhibited limb proportion and body size comparable to Homo sapiens and
to Homo erectus, and with a cranium larger than that of Homo habilis.
Comparing the African and Asian forms yields subtle but significant differences.
The skull of Homo ergaster is more generalized, having a higher or
domed cranium, fairly thin cranial bones, weak brow ridges and a lightly
built face, features that align the species more with Homo sapiens.
Alternatively, Asian Homo erectus (cover image) has
always been defined on rather specialized features including a long cranium,
a low forehead, thick cranial bones, large projecting brow ridges and a
heavier face (compared to Homo sapiens). With the new earliest dates
for Homo erectus in Asia at 1.8 mya, the two species seem to be evolutionary
contemporaries (many do not even term them separate species). Although it
is tempting to consider Homo ergaster as the more generalized African
form of Homo erectus, and therefore the first colonizer of Asia,
it is more likely that an older species ancestral to both forms left Africa
more than 2.0 mya.
Scant evidence for this pre-erectus hominid--an emergent "early"
Homo--come from four other areas of the eastern Rift in addition
to the Turkana Basin: the Hadar Basin of northeastern Ethiopia, the Omo
Valley of southwestern Ethiopia, the Baringo locality of central Kenya and
the Uraha locality of eastern Malawi. These areas have geological formations
dating to middle-late Pliocene (2.5 to 1.8 mya), the end of the critical
Pliocene cooling and the beginning of a period of climatic stability. Some
early fossils have been termed Homo rudolfensis, a taxon having affinities
with Homo habilis and possibly with Homo ergaster/erectus.
Other finds (for example, the isolated teeth from Omo and the partial temporal
bone from Baringo) are too fragmentary to classify with any specificity.
In Hadar and in Omo, early Homo is associated stratigraphically with
emergent stone-tool assemblages. Although a detailed understanding of early
Homo awaits more fossil discoveries, the significant date of origin
and the climatic link are incontrovertible.
© American Scientist 1996
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