|
ScienceWeek
HUMAN EVOLUTION: CLIMATE CHANGE AND HUMAN EVOLUTION
The following points are made by Anna K. Behrensmeyer (Science 2006 311:476):
1) Climate and biological evolution have interacted throughout Earth's history, together creating many small and a few major transformations in the planet's atmosphere and biota. The role of climate in the origin and adaptations of humans relates not only to our past but also, potentially, to our future [1]. A number of hypotheses propose that climate-driven environmental changes during the past 7 million years were responsible for hominin speciation, the morphological shift to bipedality, enlarged cranial capacity, behavioral adaptability, cultural innovations, and intercontinental immigration events [2-5]. These hypotheses are based on correlations between global-scale climate shifts documented in oceanic deposits and events in hominin evolution recorded in continental fossil-bearing strata. Establishing cause-effect relationships between climate and human evolution is tantalizing but presents many challenges for paleoanthropology and the geological sciences.
2) The biggest challenge involves how to relate different types and scales of paleoclimatic evidence between the marine and terrestrial realms. Marine-core records show that a cooler, drier, and more variable global climate regime began about 3.0 million years ago (Ma), gradually intensifying into northern continental glacial cycles by 1.0 Ma. The climate shift between ~3.0 and 2.5 Ma thus marks the onset of Northern Hemisphere glaciation, and this coincides generally with the timing of the origin of the genus Homo. Fluctuations in continent-derived dust and biomarkers in the marine record indicate that climate shifts recorded in the oceans affected the land as well.
3) However, in the continental basins that preserve hominin fossils, the record of climate change is much harder to decipher. Paleoclimatic proxy evidence includes stable isotope, pollen, mammal faunas, and lake versus land deposits. Although these signals are documented in many vertebrate fossil-bearing localities, each stratigraphic sequence represents only limited portions of the time-space framework of hominin evolution. In addition, the proxy records are subject to local tectonic and climatic processes that often obscure or completely overprint global-scale climate signals. Thus, we must confront the problem of relating a fossil record preserved in strata dominated by local- to regional-scale paleoenvironmental signals to a marine record dominated by continental- to global-scale signals. Long cores from deep African lakes could provide more continuous data and a stronger bridge between oceanic and continental climate records, but these are only beginning to be tapped.
4) Another challenge is deciding what constitutes a strong case for a causal link between a climate change and an evolutionary event. We can't step into a laboratory to test the impact of climate change on the human genome, but we do have the results of natural experiments -- the proxy evidence for environmental changes in continental rock sequences, as well as many fossils of hominins and other organisms that were evolving on different continents during that same time period. There is a rich body of data to draw upon, but hypotheses are often structured around an assumption that "synchronous" events in the geological and paleontological record constitute evidence for cause and effect. These hypotheses, while seductive in their simple explanation of how our species came to be, do not do justice to the complexity of the climate-evolution problem or to the full range of evidence and scientific methodologies that now can be brought to bear on this problem.
References (abridged):
1. National Research Council Committee on the Geological Record of Biosphere Dynamics, The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change (National Academies Press, Washington, DC, 2005)
2. L. F. Laporte, A. L. Zihlman, S. Afr. J. Sci. 79, 96 (1983)
3. S. M. Stanley, Paleobiology 18, 237 (1992)
4. E. S. Vrba, in Paleoclimate and Evolution with Emphasis on Human Origins, E. S. Vrba, G. H. Denton, T. C. Partridge, L. H. Burckle, Eds. (Yale Univ. Press, New Haven, CT, 1995), pp. 24-45
5. R. Potts, Humanity's Descent: The Consequences of Ecological Instability (Morrow, New York, 1996)
Science http://www.sciencemag.org
--------------------------------
Related Material:
ON HUMAN EVOLUTION
The following points are made by Ian Tattersall (Scientific American 2001 December):
1) When we contemplate the extraordinary abilities and accomplishments of Homo sapiens, it is certainly hard to avoid a first impression that there must somehow have been an element of inevitability in the process by which we came to be what we are. The product, it's easy to conclude, is so magnificent that it /must/ stand as the ultimate expression of a lengthy and gradual process of amelioration and enhancement. How could we have got this way by accident? If we arrived at our exalted state through evolution, then evolution must have worked long and hard at burnishing and improving the breed, must it not?
2) Yet that seems not to be how evolution works; for natural selection is not -- it cannot be -- in itself a creative process. Natural selection can only work to promote or eliminate novelties that are presented to it by the random genetic changes (influenced, of course, by what was there before) that lie behind all biological innovations. Evolution is best described as opportunistic, simply exploiting or rejecting possibilities as and when they arise, and in turn, the same possibility may be favorable or unfavorable, depending on environmental circumstances (in the broadest definition) at any given moment. There is nothing inherently directional or inevitable about this process, which can smartly reverse itself any time the fickle environment changes.
Scientific American http://www.sciam.com
--------------------------------
Related Material:
PALEOLITHIC TECHNOLOGY AND HUMAN EVOLUTION
Notes by ScienceWeek:
The evolution of the use of stone tools by ancient humans has been demarcated by anthropologists into the following stages in a time-frame from the beginning of stone-tool technology approximately 2.5 million years ago to the end of the "Old Stone Age" approximately 40,000 years ago.
a) The earliest stone-tool technology is based on simple chopping tools made by knocking several flakes off a small cobble, the "core" stone.
b) The second stage is characterized by tools that require more extensive conceptualization and preparation, such as bifacial hand-axes.
3) In the 3rd stage, large stones were preshaped by the removal of large flakes and these were then used as a source of more standardized flakes that were retouched to produce a large range of artifacts.
4) In the 4th stage, stone-tool technology was characterized by narrow stone blades struck from a prepared core.
5) The 5th stage consisted of microlith technology involving the production of small and delicate artifacts.
The above is only a general schema, with some categories overlapping when earlier technology persists after the appearance of later technology. Differences in the African and Eurasian records are believed to reflect the dynamics of the origin and migration of anatomically modern humans. Of significance, for example, is the evidence that blade tools were produced in Africa nearly 250,000 years ago, but did not enter the Eurasian record until 40,000 years ago. Categorizations are also dependent on differences in the histories of the sciences of archeology and anthropology.
The following points are made by Stanley H. Ambrose ((Science 2001 291:1748):
1) The author points out that paleoanthropologists once considered tool-making to be one of the defining characteristics of the genus Homo. However, the diversity of tool-making and tool-using behaviors among chimpanzees (Pan troglodytes) has forced a complete revision of assumptions surrounding the concept of "man the toolmaker", including revision of ideas concerning the gender of the first tool users. Chimpanzees have diverse and regionally varied repertoires of tool-using, and other "cultural" behaviors. In contrast, Cebus monkeys are considered prolific tool users but exhibit no apparent understanding of cause and effect, or of the difference between appropriate and inappropriate tools.
2) The earliest direct evidence of hominid technology dates to 2.5 million years ago in the Ethiopian Rift Valley, the artifacts including sharp-edged slivers and lumps of stone, hammer stones and anvils, and bones with hammer marks and cut marks from butchery and marrow extraction. This simple technology is called the "Oldowan Industrial Complex", after excavation localities in Olduvai Gorge, Tanzania. Early hominids apparently possessed an excellent empirical understanding of the mechanical properties of lithic raw materials, fracture mechanics, and geometry.
3) *Homo habilis is usually considered the first tool maker. Cranial internal-cavity casts (endocasts) of these fossils show that its left brain hemisphere has an impression of Broca's area, the cortical area involved in speech and language, an area that is adjacent to and probably derived from the area for precise hand-motor control. The author points out that approximately 90 percent of humans are right-handed, and hand preference is strongest in skilled tool use involving a precision grip. Individual chimpanzees exhibit long-term consistency of hand preference mainly for complicated tool-using tasks, but there is no overall preference among chimpanzees for right-handedness at the population level.
4) Large cutting tools, typically approximately 10 to 17 centimeters long, were apparently added to the Oldowan toolkit approximately 1.5 million years ago, and this marks the advent of the so-called "Archeulean Industrial Complex". Archeulean technology is associated with the fossils of H. erectus and H. heidelbergensis, in the time-frame 1.5 to 0.3 million years ago. Large flakes, slabs, and cobbles were shaped into large cutting tools by bidirectional or unidirectional invasive trimming of lateral edges. Discovered hand-axes from this period typically have a tear-drop-shape and a lenticular cross-section. Cleavers have a sharp, thin, usually unmodified edge transverse to the long axis. Picks and knives have convergent tips, like hand-axes.
5) Technological and cultural evolution accelerated approximately 300,000 years ago, during the Middle Paleolithic period in Eurasia, and during its sub-Saharan African correlate, the "Middle Stone Age". These advances were made by Neanderthals, by late archaic humans, and by anatomically modern humans. Regional stylistic and technological variants are clearly evident, suggesting the emergence of true cultural traditions and culture areas. Large cutting tools were supplanted by smaller tools, and there is evidence of a sophisticated technology for producing relatively standardized artifacts, which may reflect more complex cognitive abilities. Stone-tipped spears, knives, and scrapers mounted in shafts and handles represent a significant increase in technological complexity.
6) Although blade-based lithic technologies occurred throughout the Middle Paleolithic period, more sophisticated technologies appeared approximately 50,000 years ago in East Africa and the Levant. Blade production substantially increased the number of usable sharp edges that could be obtained from a core. Standard blade blanks were shaped into a diverse array of functionally and stylistically distinct tool types, often as components of tools of greater complexity. Of greater significance are ground, polished, drilled, and perforated bone, ivory, antler, shell, and stone, shaped into projectiles, harpoons, buttons, awls, needles, and ornaments. Such artifacts are extraordinarily rare in Middle Paleolithic sites, but are a consistent feature of Upper Paleolithic and Later Stone Age sites after 40,000 years ago.
7) The author concludes: "Did the challenges posed by the increasingly variable, severe, and risky environments of glacial/interglacial cycles over the past 800,000 years, as well as more dramatic short-term climatic events, influence behavioral and biological evolution? Or were changes increasingly autocatalytic, driven by language and by cultural systems of knowledge and understanding of nature and society? With the appearance of near-modern brain size, anatomy, and perhaps of grammatical language approximately 300,000 years ago, the pace quickens exponentially... A mere 12,000 years separate the first bow and arrow from the International Space Station."
Science http://www.sciencemag.org
--------------------------------
Notes by ScienceWeek:
Homo habilis: In 1964, an early fossil *hominin (1.9 to 1.6 million years before the present) was found in Olduvai, Tanzania, the brain apparently intermediate in size between the earliest known Homo fossil *Homo erectus and the *Australopithecus group. This new fossil was denoted as a new species by its discoverers and named Homo habilis. The original set of H. habilis fossils included a relatively complete hand, its structure apparently compatible with an ability to make and use tools. (Homo habilis literally means "handy-man".) Considerable controversy in the paleoanthropology community concerning H. habilis has continued from 1964 until the present.
hominin: In general, any human-related fossil group.
Homo erectus: First discovered by Eugene Dubois in 1891 in Indonesia, this fossil group is currently viewed as the closest precursor to H. sapiens. Formerly called "Anthropithecus erectus" and "Pithecanthropus erectus". Pithecanthropus erectus and Sinanthropus erectus ("Peking man", discovered in 1927) were in 1951 subsumed under the single category Homo erectus, which was then recognized as a widespread species exhibiting significant geographical variation.
Australopithecus: A now extinct genus believed to have existed between 4.4 and 1 million years ago, and believed to have been precursors of the genus Homo. All australopithecines are apparently characterized by an ape-like form, rather than the human-like form of the Homo genus.
ScienceWeek http://scienceweek.com
|