By Robert Roy Britt Senior Science Writer From Space.com, 24 April 2001 In the 5 million years or so that it took for apes to become human, many human-like branches of the evolutionary tree were lopped off. Scientists have long wondered why these other hominid species, estimated to number a dozen or more, didn't make it. Were those who came to travel to the Moon and ponder their very origin the logical and inevitable victors in the most important of all Darwinian struggles? Or did we just get lucky? A newly presented mathematical argument suggests that the birth of Homo sapiens was guided by catastrophic asteroid or comet impacts, which created climate conditions that competing species, frankly, couldn't handle. It also holds that our human ancestors avoided early elimination by the statistical skin of their rotting teeth. "The reason that Homo sapiens have survived in spite of these global disasters has little to do with the traditional explanations given by neo- Darwinists," said Benny Peiser, a social anthropologist at Liverpool John Moores University. "It is sobering to realize that we are alive due to cosmic luck rather than our genetic makeup." Peiser bases his argument on the fact that populations of hominids and early modern humans were extremely small. "Had any of these impacts occurred in the proximity of these population groups, we might also have gone the way of the dodo," he said. The study's assumptions and calculations have met with strong caution and even sharp criticism among scientists who specialize in evolution, as well as asteroid experts. Adaptive advantage David Balding, a professor of applied statistics at University of Reading in the U.K., said the idea that human survival is due to "cosmic luck" does not compute: "Perhaps we were lucky in avoiding a massive impact, but perhaps it was our adaptive advantage that helped us survive modest regional impacts whereas our hominid cousins did not," said Balding, whose own research focuses on human evolution. But some called the new scenario plausible. It has not been published in a peer-reviewed journal but it is based on impact estimates that are generally accepted by the asteroid research community, though there are disagreements over the precise number of times a large asteroid or comet has struck our planet. Peiser laid the idea out earlier this month at a conference, "Celebrating Britain's Achievements in Space." He worked with Michael Paine, a volunteer for the Planetary Society in Australia who ran impact scenarios through a computer program. (Paine has written freelance stories for SPACE.com in the past.) The researchers concluded that there would have been 20 "globally devastating" impacts during the past 5 million years, with effects strong enough to have had "a catastrophic and detrimental effect" on human evolution. Five million years ago is roughly the time when hominids diverged from other apes, though some recent controversial evidence puts the split as far back as 6 million years ago. Did space rocks set the human stage? No one argues that asteroids can be devastating when they tangle with Earth. An impact 65 million years ago is widely believed to have spurred the demise of the dinosaurs and many other animals and plants. But efforts to tie other, more ancient mass extinctions to impacts remain inconclusive. While extinctions are clearly identifiable in fossil records, impact evidence seems not to survive the millennia as well. So impact estimates are based largely on the Moon -- a nearby archive of countless craters that have not eroded much over time. Still, because scientists have not witnessed a severe impact, the presumed effects are speculative. If an asteroid larger than a kilometer (0.6 miles) hit the planet, it would cause instant death across a wide area near the site of impact, and researchers generally agree that drastic climate changes could last a year or more. Even our protective ozone layer could be damaged, studies have shown. But the precise consequences of these effects are not known. It is thought that long-term climate change could make life impossible for many species, which in turn would cause mass death that might move up entire food chains. Peiser suggests another possible effect: "The abrupt loss of the ozone layer and the sudden release of toxins may even affect the DNA in some unknown manner, thus triggering macro- mutations, including the sudden reorganization of entire genomes." Ellen Thomas, a Wesleyan University research professor who examines how climate change affects evolution, said few evolutionists would by this argument of quick, significant changes in the genetic blueprints. Instead, macro-mutations are seen by many as a genetic dead end. "Macro-mutations can hardly ever lead to evolution," Thomas said. "They lead to non-viable organisms." Basic numbers questioned Of course to affect human evolution in any fashion, a space rock first has to hit Earth. But "no one knows how many impacts took place, or when, or with what severity, over the past 5 million years," said David Morrison, an asteroid expert at NASA's Ames Research Center in California. Morrison told SPACE.com that instead of the 20 potentially devastating impacts assumed by the study, he expects there were probably only five or 10 with enough energy to create global environmental effects. "But we know very little about specific impacts in this time frame, and virtually nothing at all about their actual environmental effects," Morrison said, adding that there is "no evidence of an impact associated with a hominid extinction." Morrison did not discount the whole idea, however. "I would be surprised if impacts had not had some influence on early hominid populations and perhaps evolution," he said. "On the other hand, I am not convinced that impacts led to numerous extinctions in the past 5 million years. This is all interesting speculation, but specific data are lacking on either impacts or extinction events and there is no known correlation between the two." Peiser counters that the estimates used in the study are "very conservative." He acknowledges that shortcomings in the human fossil record (fossils on land erode more easily than those in the oceans) "are far too big to allow any direct correlation between impact catastrophes and hominid extinction." But he said that the study shows that "impact catastrophes that occurred during the crucial period of human evolution should no longer be ignored." Still, it is clear that more research will be needed before any consensus emerges. "What [Peiser and Paine] may have added," said Balding, the statistics professor, "is some quantitative simulations to make more precise some well established speculations." Speculation about evolution is nothing new. And the more one delves into the nitty-gritty of our own past, the stronger the criticism gets over Peiser's attempt to reinvent Darwin. Does Darwin need reinventing again? If asteroid experts are sometimes a mile apart on their view of history -- and they are -- then evolutionary theorists live on different continents. Followers of Charles Darwin have long believed that failed branches of our ancestry reflect a common mode of evolution, whereby species are gradually replaced by more advanced species that adapted because of their superior genetic fitness. But in recent decades, a different view called punctuated equilibria has taken hold. This theory, first put forth in the 1970s by Stephen J. Gould and Niles Eldredge, expects sharp changes in evolution. In either scenario, luck plays a role. And both fit within the most famous of Darwinian themes, survival of the fittest. But the rapid shifts assumed in punctuated equilibria, be they caused by sudden disasters or other means, are thought to be the mechanism by which one species replaces another. "There has been debate for over 100 years on whether evolution is gradual or punctuated," said Balding. And the debate continues. Recent fossil findings have some researchers leaning back toward the gradual approach to human evolution. Peiser said his study supports punctuated equilibria, and helps explain why "almost all hominids, i.e. the 14 known species of human ancestors, have become extinct during the last 5 million years." But Wesleyan's Ellen Thomas said it is not even known that there were 14 species. "The human fossil record is incomplete, and it is not easy to agree on which fossils belong to different species," Thomas said in an e-mail interview. "The experts disagree wildly." Thomas echoed other scientists in pointing out that there is no fossil evidence -- neither of human remains in Africa nor marine organisms, which leave a much more complete record -- that reveal any mass die- offs during the 5 million-year period covered in Peiser's study. "And if the extinctions affected humans, they should show up in the extinction record of other organisms as well," Thomas said. "The paper just shows that many impacts, many of which could have been damaging, possibly occurred." But Peiser argues that no expert on near-Earth asteroids, the space rocks known to exist in our region of the solar system, questions that "many such global disasters must have occurred." Yet he said "all textbooks on human evolution completely ignore the occurrence of catastrophic impacts." Other forces of evolution While Peiser and Paine suggest that comets or asteroids are a driving force behind evolutionary change, it is the climatic consequences of impacts that are the would-be crushing mechanisms for fledgling species. Other researchers have long debated possible links between climate change and human evolution. For example, cold periods are suspected of forcing migrations that created small, isolated groups that could have evolved significantly but then died out. One such period may have occurred as recently as 71,000 years ago. But firm links between climate and serious evolutionary changes elude researchers. One recent international study, released earlier this year and led by Jeremy Marlow of Newcastle University, showed evidence of a significant cooling of the climate 2 million years ago that the authors said "adds weight to the theory that climate change played a significant part in the evolution of early humans." Further clouding the possibilities, recent findings have hinted at the possibility that the worst extinctions might require multiple killing mechanisms, such as when an impact, or perhaps several, happens to occur during a time of heavy volcanic activity. Irony in our existence In an ironic preface to the whole argument, it's possible that asteroids and comets were responsible for life in the first place. A growing movement among astrobiologists suggests that rocks from space brought critical building blocks that stimulated the initial biological activity in the earliest primordial soup billions of years ago. But regardless of whether cosmic messengers helped make us who we are, there is one thing researchers seem to agree on: Given the evidence that our ancient ancestors were clustered in a relatively small area (in Africa) you are somewhat lucky to be reading about all this. "Asteroids certainly had the opportunity to wipe out man at his roots," said Jack G. Hills, an asteroid specialist at Los Alamos National Laboratory. "Only good luck prevented it."
Liverpool John Moores University Press Office Janet Martin 0151 2313583 j.martin@livjm.ac.uk Press Release 17 April 2001 The theory of gradual and uninterrupted human evolution has been called into question after two researchers found that human evolution has been repeatedly punctuated by large-scale cosmic catastrophes. One of the biggest problems that has puzzled researchers for generations is the question why almost all hominids, i.e. the 14 known species of human ancestors, have become extinct during the last 5 million years. Ever since Charles Darwin, the predominant explanation has been that our ancestors were slowly and gradually replaced by more advanced species due to their inferior genes and their failure of adaptation, and that modern humans survived due to their superior "fitness". In recent years, however, researchers have become aware that the fossil record does not show gradual but rather abrupt change. Neither the causes for the sudden gaps in the fossil record nor the underlying dynamics of hominid extinctions have been determined yet. Dr Benny Peiser, a social anthropologist at Liverpool John Moores University and Michael Paine, an impact researcher from the Planetary Society in Australia, have come up with new findings that may help to solve this puzzle. Peiser and Paine have calculated that the most likely cause of hominid extinctions may be associated with the more than 20 globally devastating catastrophes that occurred over the last 5 million years. Dr Peiser presented the findings last Thursday at the Charterhouse Conference 2001 "Celebrating Britain's Achievements in Space". It is these large-scale impacts that had a catastrophic and detrimental affect on human evolution. A number of significant climatic downturns, well documented in the paleo- environmental and climatological records are directly associated with such major impact. Episodes of cometary or asteroidal impacts punctuating human and societal evolution should thus be looked upon as natural agencies determining evolutionary regressions, extinctions and macro-mutations. "The reason that Homo sapiens have survived in spite of these global disasters has little to do with the traditional explanations given by neo- Darwinists. It is sobering to realise that we are alive due to cosmic luck rather than our genetic makeup," says Dr Peiser. "After all, the populations of hominids and early modern humans were extremely small. Had any of these impacts occurred in the proximity of these population groups, we might also have gone the way of the dodo." Planetary Society's Michael Paine agrees: "Just over 2 million years ago an asteroid estimated to be 2km in diameter struck the Southern Ocean, south west of Chile. Had it struck land the environmental consequences might have been much worse. If the collision had occurred a few hours early southern Africa might have been wiped out, along with our ancestors." Large impacts not only cause severe climate disruption (mainly darkness and cooling) but can also lead to the loss of the ozone layer (particularly with ocean impacts that propel chlorine into the upper atmosphere), severe acid rain and toxins. "Such environmental mega- catastrophes may not only wipe out directly affected hominid species. What is more, the abrupt loss of the ozone layer and the sudden release of toxins may even affect the DNA in some unknown manner, thus triggering macro- mutations, including the sudden reorganisation of entire genomes," Dr Peiser suggests. Given the rather conservative impact estimates, "it is almost certain that, for several years at least, creatures on Earth had to endure very severe conditions. Although the physical Earth heals quickly from impacts its inhabitants might not do so well and the course of evolution may well have changed due to these and similar impact events," says Michael Paine. Dr Benny Peiser b.j.peiser@livjm.ac.uk 0151 231 4338 (office) Michael Paine mpaine@tpgi.com.au +612 94514870
Accepting the inevitability that Earth is struck by extraterrestrial objects and that there are statistics that give pointers to impacts in terms of energy and frequency, I feel that there is a long way to go in assigning speciation (as opposed to general mass extinctions) to external causes. Speciation among our ancestors is particularly fraught with difficulty, because of the blurred record. It is blurred in several ways: 1. By the tiny number of fossils 2. By the dates assigned to those fossils 3. By the significance assigned to their morphology by different palaeoanthropologists - there are "lumpers" and "splitters" 4. By the total lack of knowledge about the interplay between physiology, culture and social interaction, as regards what constituted "fitness" in natural selection. This sets our own record apart from those of other species, as does the uniquely wide dispersion of hominids throughout the "Old World" from about 1.8 Ma. Evolutionary bottlenecks are indeed implied by extant human genetics - mtDNA and Y-chromosome data - at ca 200 ka and 70 ka respectively. Beyond that there is simply not a hint. The cultural and morphological stasis of H. erectus for 1 Ma (represented by the greatest number of fossils and artifacts before about 400 ka) could be read as "evidence" for a lack of such bottlenecks in the 1.4 to 0.4 Ma period. Using impact statistics is a means of assessing the probabilities of extraterrestrial events over various time spans, but that does not allow any kind of prediction of precisely when events of different magnitudes might have taken place. That only emerges from evidence in the form of craters which have been dated precisely, which are thin on the ground, despite discoveries of geologically young craters. The record of environmental change at the global level lies in the oxygen and carbon isotope time series from ocean- floor sediments, supplemented by polar ice-core records since about 425 ka. This has a precision of better than +/- 200 years for the last 125 ka, degrading to +/- 4 ka at the Oligocene- Miocene boundary (~23 Ma ago). The record is indeed "punctuated" by major climatic shifts (about 50 over the last 2.5 Ma) and a very large number of lesser ones in the high-precision time series since 425 ka. The idea that these time series stem from variations in solar input modulated by changing orbital obliquity, axial tilt and precession, which are conditioned by oceanographic and atmospheric factors, is probably the most impregnable in the Earth sciences, simply because of the quality of the data and their analyses. While there may be climatic events that buck this model, they do not have the amplitude to interfere with it. Because the record of human ancestry is so patchy no-one to my knowledge has seriously considered linking speciation to these known climatic events, except in the most general way (eg de Menocal, P.B., Science 1995, v. 270, p. 53-59). To take the influence of impacts on recent biological evolution from speculation based on rather wooly statistics to being worthy of serious consideration needs several developments: Detection of "spikes" in the ocean- floor and ice-core records that cannot be explained by a blend of the Milankovich model with oceanographic and armospheric factors, or by volcanicity. The best time series for that would be those bearing on dust and acid rain from electrical conductivity in ice cores, and carbon isotopes in ocean-floor cores that link to biological productivity. Examination of the full terrestrial fossil record of other animals over the last 5 Ma to seek otherwise inexplicable speciation events and minor, general extinctions. It is not a case of the "jury being out", but of the evidence not being in - it is meagre in the extreme. Steve Drury Dept of Earth Sciences The Open University
Marco Langbroek Palaeolithic Archaeologist Leiden University, the Netherlands With interest I have read NEO News 4/24/01 on Impacts & human evolution, and I have a few points to say about this. I am a professional archaeologist (Leiden University, faculty of Archaeology) working in the field of early human evolution, as well as a high level amateur meteoriticist. I therefore know both sides of the coin (human evolution & meteoritics) rather well (for example, I am co-author to a paper on an earth- threatening comet: P. Jenniskens et al., ApJ 479 (1997), 441- 447), and certainly the Human evolution side. Moreover, I have been pointing to a possible significant impact with regard to early human presence in Southeast Asia in a paper published a year ago. As a side remark in that paper (co- authored by Wil Roebroeks) on the chronology of the earliest human occupation of Southeast Asia, which appeared in April 2000 (M. Langbroek & W. Roebroeks: J. Human Evolution 38 (2000), 595-600), in which tektites were an argument, I have pointed out that a large impact, believed by some to be one of the largest of the past few million years, occurred in SE Asia 0.8 Ma ago, and that it was of such a magnitude that it must have had serious consequences for the palaeo- environment and biogeographical history (perhaps including local hominid evolution) of Southeast Asia. This topic is discussed in more detail in my 1998 Master dissertation, and will be discussed in more detail in an appendix to my upcoming PhD dissertation. The impact in question is the impact which created the large Australasian tektite strewnfield. It is extremely well dated at 0.80 Ma (see our paper and refs therein). This is well within the time span of human evolution, and moreover, humans might have been present near or in the actual impact affected area. The impact is believed to have occurred in the area of Laos, Cambodia or Thailand, from multiple lines of evidence. Schmidt & Wasson (Meteoritics 28 (1993), 430) have, from the strewnfield structure, estimated the strewnfield to be caused by an impact with an energy release in the order of 5x10^4 to 1x10^5 MT. In the impact hazard assessment model of Chapman and Morrison (Nature 367 (1994), 33-39), this would be an impact at the lower limit of the global effects threshold . Apart from possible short term effects due to atmospheric dust release, ozone depletion and acid rains, employing the simple scaling relationships from the Chapman and Morrison paper also shows that an area of about 150 000 to 200 000 square kilometer, or an area 500 kilometer in diameter, would be directly affected by air blast phenomena. Such an area is significant in size: it amounts to as much as about one third of the longitudinal diameter of the Southeast Asian peninsula/Sunda shelf at this location. In a larger area of Indochina roughly 1000 km in size, impact ejecta in the form of large Muong-Nong type tektites occur. These are believed to have been still plastic and hot upon landing (Fiske, Met. Plan. Sci. 31 (1996), 42- 44), creating the possibility that they started wildfires within this large area which covers a significant part of the Indochinean peninsula. This is considerable ecological havoc on a sub- continental scale. We know that hominids were present in China at about 1.1-1.2 Ma, well before the impact. Debates are currently raging (and I am in the thick of it) concerning the earliest occupation of Southeast Asia. Some would have it as early as 1.8 Ma, which I strongly doubt (see our 2000 JHE paper). But even my more conservative estimate brings humans in Indonesia at about 1.1 Ma, which still is before the impact. To get to Indonesia, they had to cross the area of impact first, hence the impact area would probably have been settled in a broad sense. Yet evidence from the actual impact area (Cambodia- Laos,Vietnam, Thailand) dating from the time of impact is actually absent. This seems to have a geological reason as not only traces of human presence, but faunal assemblages in general are lacking from this area from this time span: in addition, there is the enigmatic point that the impact crater itself has not been positively identified yet. This indicates that deposits from this time span have become inaccessible due to (probably) some geological reason. Uncertainties concerning the chronology of Quaternary Sunda make it currently impossible to tie biogeographic events on the Sunda shelf south of the impact area with possible effects of the impact. While very significant on a sub- continental scale (which is thus not to be ignored!) and perhaps (and inevitably if they indeed were present in the actual impact affected area) leading to the extinction of a local subgroup of Homo erectus, there is NO evidence that this led to long term effects in whatever way, notwithstanding that this was one of the largest impacts in the time span of human evolution. Detailed study of Deep Ocean Drilling Core data by Schneider, Kent & Mello (Earth Planet. Sci. Lett. 111 (1992), 395- 405) revealed no evidence for significant climatic perturbations on a timescale of 1 ka: compared with the strong effects of glacial cycles, this impact was hardly a ripple in the ocean. Likewise, even if local Homo erectus inhabitants of the impact area were wiped out (as seems likely) on a sub continental scale, this seems to have had little evolutionary effects. Once the dust settled, the population was simply restocked by Homo erectus from outside the impact-affected area. Such abandonment and repopulations of areas probably were common in the Pleistocene, certainly in this area, connected to events that had nothing to do with impact but where of similar scales or even scales surpassing the short-term effects of impact: for example the strong effects of cyclical flooding and re- emergence of the Sunda shelf just south of the impact area, due to the cycles of sea- level change connected to the glacial cycles: or the known waxing and waning of the Human population of Europe over time (e.g. the depopulation of large parts of Europe during the height of the last glacial 18 000 years ago) as a result of the glacial cycles. This brings me to a concern with the picture such as painted by Peisser and Paine. I am convinced that only the largest of impact would have had potential to seriously shape Human evolution: and with that I am thinking of the very rare impacts of K/T size, which are not likely to have happened the past 5 to 8 million years of hominine evolution (and at any rate, such an impact would have yielded clearly discernable signs in the geological and palaeontological record, in the form of mass extinctions of a considerable part of the global fauna and flora, as well as widespread (=global) presence of impact ejecta). Any smaller impacts will simply have had effects which were insignificant compared to the powerful driving force of the glacial cycles of the last few million years. Effects would have been very local and temporary when seen in that context, being hardly more than ripples in the ocean, whereby an area was temporarily depopulated and then repopulated again. Where climatic effects (and by inference ecological effects) stay short-term and do not wipe out significant parts of the global or continental biomass (and there is no evidence for such non- short term, non- local effects due to impact in palaeoclimatological and biochronological proxy data of the past million years, while these effects should be discernable if they happened), evolutionary effects are very minor and such things will not lead to profound speciation. As Steve Drury very rightly pointed out in his comment, Peisser and Paine make a mistake in focusing on hominid EXTINCTION: what really drove human evolution was an unbound hominid SPECIATION during the Pliocene and Pleistocene. In the absence of a near continental ecological wipe-out, which we know did not happen, such phenomena can come about only through long term trends of ecological diversification on a large geographic scale, as happened in Africa when a trend of aridification and increasing seasonality set in at the end of the Miocene, with some profound accelerations near 2.5 and 1.8 Ma, creating long term change in the ecology and new directions of ecological developments. In this sense, I am a bit afraid that Peisser and Paine seem to be ignorant of what evolution really amounts to. The extinctions they focus upon where an unimportant side effect created by speciation: hominine species only could become extinct because they had been created by speciation first, and when ecological diversification due to long term trends in ecological change creates speciation events, there inevitably have to be a series of extinctions following too. Many of the large number of hominine species Peisser and Paine point out got extinct during the Pliocene and Pleistocene, were species that were chronologically contemporary, sharing a similar time span and in general a similar geographical area and grosso modo similar structured ecology: but filling slightly differing niches within that grosso modo similar ecology and geographic area. It were long-term changes in the details of ecological structure which drove evolutionary selection: while on the other hand an impact of the necessary continental scale (Pliocene African Australopithecines at 3.5 Ma for example where present in an area ranging from South Africa via East Africa to Chad!)would have wiped them out all at once, along with many other mammalian species which emerged and evolved during the same time span in the same area due to the same evolutionary processes. That should leave its signs in the record, and the signs are not there. Therefore, I really strongly doubt whether cosmic impacts played a major significant role in Human evolution of the past 5 to 8 million years. Marco Langbroek Faculty of Archaeology Leiden University P.O. Box 9515 NL- 2300 RA Leiden the Netherlands |
