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Human Dispersal and Late Quaternary Megafaunal Extinctions: the Role of the Americas in the Global Puzzle

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As Alfred Russell Wallace once wrote, we live in a zoologically impoverished world, from which most of the largest, strangest and most spectacular animals disappeared quite recently. About two thirds of all animal species larger than 50 kg (the so-called megafauna) were extinct from the late Pleistocene onwards, starting in Australia at about fifty thousand years ago and following humans' footsteps is their expansion throughout Eurasia and the Americas. The extinctions went on through the Holocene, reaching islands all around the globe, that can be seen as 'time machines' where megafauna survived for millennia after the continental extinctions, such as the Caribbean, the islands off Alaska, and Wrangel Island in the Arctic Ocean. In Madagascar and New Zealand, extinctions are but a few centuries old. These late Quaternary extinctions were a global phenomenon that begs for a global explanation. Climatic hypotheses fail to explain these patterns for several reasons, for example, there were dozens of other glacial cycles throughout the Pleistocene, without associated mass extinctions; extinctions in Australia and the islands did not coincide with glacial peaks; and climate changes cannot explain why extinctions were systematically more recent on islands. However, the pieces of the puzzle immediately fit together when we observe the clear correspondence between the dates of humans' arrival and of megafaunal extinction in each landmass. Bernardo Araujo recently analysed the chronology of extinctions of megafaunal genera around the world. He found that extinctions took place closer than expected by chance to periods of high climatic variation alone in only two of the analysed cases, to dates of human arrival alone in seventy-four cases, and to both in eight cases, with 40 cases unexplained. Thus, anthropogenic impact is the most plausible and parsimonious main cause of the late Quaternary extinctions. In a modern view, the extinctions were a long process that took several millennia to occur in most continents, with a few stragglers like the Irish elk and the North American mastodons. Low reproductive potential was the main determinant of the extinct species; the apparent selection by size is an artefact of the inverse correlation between the two variables. The absence of evolved instincts against newly arrived humans, the difficulty of conserving meat and the lack of perception of the world's finitude must have contributed to the outcome. Thus, human-megafauna interactions are an important and undervalued part of human history that merits being represented on the UNESCO World Heritage List. Furthermore, learning from the extinctions of the past is crucial to allow us to minimise extinctions in the future. Candidate sites in the Americas might include those that show consumption of megafauna (such as Monte Verde), remarkable rock paintings (such as Serra da Capivara, Brazil) and the latest American megafauna (such as Las Breas de San Felipe, Cuba).
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A Genetic and Biological Perspective of the First Settlements of the Americas
Human Dispersal and Late Quaternary Megafaunal
Extinctions: the Role of the Americas in the Global Puzzle
Fernando A. S. Fernandez
Departamento de Ecologia – Universidade Federal do Rio de Janeiro – Brazil
Abstract
As Alfred Russell Wallace once wrote, we live in a zoologically impoverished world, from which most of the
largest, strangest and most spectacular animals disappeared quite recently. About two thirds of all animal
species larger than 50 kg (the so-called megafauna) were extinct from the late Pleistocene onwards, starting
in Australia at about fifty thousand years ago and following humans’ footsteps is their expansion throughout
Eurasia and the Americas. The extinctions went on through the Holocene, reaching islands all around the globe,
that can be seen as ‘time machines’ where megafauna survived for millennia after the continental extinctions,
such as the Caribbean, the islands off Alaska, and Wrangel Island in the Arctic Ocean. In Madagascar and New
Zealand, extinctions are but a few centuries old. These late Quaternary extinctions were a global phenomenon
that begs for a global explanation. Climatic hypotheses fail to explain these patterns for several reasons,
for example, there were dozens of other glacial cycles throughout the Pleistocene, without associated mass
extinctions; extinctions in Australia and the islands did not coincide with glacial peaks; and climate changes
cannot explain why extinctions were systematically more recent on islands. However, the pieces of the puzzle
immediately fit together when we observe the clear correspondence between the dates of humans’ arrival and
of megafaunal extinction in each landmass. Bernardo Araujo recently analysed the chronology of extinctions
of megafaunal genera around the world. He found that extinctions took place closer than expected by chance
to periods of high climatic variation alone in only two of the analysed cases, to dates of human arrival alone
in seventy-four cases, and to both in eight cases, with 40 cases unexplained. Thus, anthropogenic impact is
the most plausible and parsimonious main cause of the late Quaternary extinctions. In a modern view, the
extinctions were a long process that took several millennia to occur in most continents, with a few stragglers
like the Irish elk and the North American mastodons. Low reproductive potential was the main determinant
of the extinct species; the apparent selection by size is an artefact of the inverse correlation between the two
variables. The absence of evolved instincts against newly arrived humans, the difficulty of conserving meat and
the lack of perception of the world’s finitude must have contributed to the outcome. Thus, human-megafauna
interactions are an important and undervalued part of human history that merits being represented on the
UNESCO World Heritage List. Furthermore, learning from the extinctions of the past is crucial to allow us to
minimise extinctions in the future. Candidate sites in the Americas might include those that show consumption
of megafauna (such as Monte Verde), remarkable rock paintings (such as Serra da Capivara, Brazil) and the
latest American megafauna (such as Las Breas de San Felipe, Cuba).
Introduction - the late Quaternary extinction sequence
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sense of incompleteness felt by Wallace, at a time when palaeontology was still a young science, is well justified. In
the last 50,000 years - the blink of an eye in geological times - the planet has lost an impressive array of large animals,
including mammoths, mastodons, woolly rhinos, sabertooth tigers, giant ground sloths, gliptodonts, moas, elephant
birds, marsupial lions and many others. These recently lost beasts represented about two thirds of all the world’s
OGICHCWPCJGTGFGHKPGFCUCPKOCNUYKVJCFWNVDQF[YGKIJVCDQXGMI(QTGZCORNGCVNGCUVQHIGPGTCQHNCTIG
mammals were lost between 50 thousand years ago (henceforth ky) and 500 years ago (Turvey and Fritz, 2011). When
trying to understand the causes of this huge ‘prehistoric revolution’ (Martin and Klein, 1984), it is crucial to note that
these extinctions were not an event well-defined in time, but rather a process: the megafauna disappeared at markedly
different times in different parts of the planet. We will briefly review this dramatic sequence.
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A Genetic and Biological Perspective of the First Settlements of the Americas
The extinctions first reached Australia, around 50 ky. By that time, the spectacular Australian fauna included the diprotodonts
(genus Diprotodon), giant marsupial browsers that could weigh almost three tons; Palorchestes, ecological equivalents to tapirs;
Thylacoleo carnifex, the ‘marsupial lion’ (actually a leopard-sized large carnivore) and a whole family of giant short-nosed
kangaroos, the Sthenurinae (Murray, 1984; Johnson, 2006). Aside from the mammals, there were gigantic reptiles such as
Megalania prisca that resembled an oversized Komodo dragon, Wonambi naracoortensis, similar to a large anaconda, plus
some huge terrestrial birds, Genyornis newtoni. The Tasmanian ‘wolf’ or thylacine, Thylacinus cynocephalus, often considered
endemic of Tasmania (where it survived until 1936), was also found throughout Australia at that time. In continental Australia,
the extinctions took place during an ‘extinction window’ that went from 50 to 40 ky, with a peak at around 46 ky (Roberts et
al., 2001). A recent claim by Wroe et al. (2013) that the extinctions in Australia were spread in time since the Middle Pleistocene
does not seem consistent, because the species at that time were nearly all from the same genera as the ones that disappeared
during the ‘extinction window’, and thus they are likely to represent taxonomical splitting rather than a truly different species
set. An interesting point is that in Tasmania the extinctions happened distinctly later than in continental Australia, around 41
to 40 ky (Turney et al., 2008).
In Eurasia, the extinctions were quite spread out in time, roughly presenting two pulses (Turvey and Fritz, 2011). In the southern
latitudes, around the Mediterranean Basin, most species vanished between 45 and 20 ky. The extinctions in the northern
latitudes came much later, from 14 to about 9 ky. Some stragglers survived even later, such as the ‘Irish elk’ (Megaloceros)
in the Ural Mountains until some 7,700 years ago (Stuart et al., 2004). Among the main losses suffered in Eurasia were the
Eurasian mammoths (Mammuthus primigenius), woolly rhinoceros (Coelodonta antiquitatis) and cave bears (Ursus spelaeus).
The coexistence of humans with this striking fauna, including the hunting activity, is richly documented in the superb Cro-
Magnon rock paintings, in sites such as Altamira, Lascaux and El Pindal. This coexistence lasted until the big animals became
extinct, soon followed by their artistic representations.
In North America, the extinction process was comparatively ‘quick’, as it took place mostly between 13.5 ky and approximately
11 ky. Again, some stragglers survived until more recent times, such as the mastodons (Mammut americanum), whose last
records, in the region of the Great Lakes, are dated to 9,900 years ago (Woodman and Athfield, 2012). The North American
megafauna at this time was very rich and much more similar to Africa’s present fauna than we could imagine, based on the
remaining species there (Anderson, 1984). Among others, there were North American mammoths (Mammuthus columbi),
mastodons, giant ground sloths (see next paragraph) and also camels (Camelops hesternus), horses and bison, including
the huge long-horned bison (Bison latifrons). Furthermore, North America was home to an exceptionally rich carnivore
fauna, including the giant bear (Arctodus simus), the sabertooth tiger (Smilodon fatalis), the only slightly smaller scimitar
cat (Homotherium serum), lions (Panthera leo atrox), oversized wolves (the dire wolf, Canis dirus) and cheetahs (Miracinonyx
trumani) (Flannery, 2001).
In South America, the extinctions were spread along a longer period of time than in North America, from about 13 ky to 7.8
ky (Barnosky and Lindsey, 2010). This means that, although there is a considerable overlap in time between the continents,
many of the South American extinctions took place after the process was mostly completed in North America. The South
American megafauna, like their North American counterparts, also included at least two species of elephants (from the
genera Haplomastodon and Cuvieronius), sabertooth tigers (Smilodon populator) and a rich diversity of ground sloths (families
Megatheridae, Megalonychidae, Milodontidae and Nothotheridae). The largest ground sloths (genera Megatherium and
Eremotherium) were huge, slow animals that could weigh more than five tons. Besides, there were large native grazers similar
to hippos called toxodonts (Toxodon platensis), many gigantic armadillo-like gliptodonts and a rich diversity of roofed beasts
including highly peculiar animals like Macrauchenia as well as more familiar horses (Equus). Although today South America is
relatively poor in megafauna - the tapirs, jaguars and marsh deer are the few remaining large mammals - this was not by any
means true about 15 thousand years ago.
The extinctions were not limited, by any means, to the Pleistocene-Holocene boundary - on the contrary, they went on through
the Holocene. That is why it is more correct to talk about ‘late Quaternary extinctions’ rather than using the misleading
expressions ‘Pleistocene extinctions’ or ‘Pleistocene-Holocene extinctions’, the latter seeming to imply that they happened
in the boundary between the two epochs. It was indeed well into the Holocene that the extinctions finally reached some
‘time machines’ where some last members of the vanished megafauna still survived. These time machines –places were intact
Pleistocene ecosystems could still be found well into the Holocene - were the islands.
In the Caribbean, all several dwarf giant ground sloths became extinct from about 6 ky to 4.7 ky, at least some 4,500 years later
than anywhere in the mainlands of either North or South America (Barnosky et al., 2004; Steadman et al., 2005). The most
recent records are from Cuba, the largest island in the Caribbean. In Beringia, mammoths (Mammuthus primigenius) survived
on the island of St Paul (near Alaska) (Guthrie, 2004) and Wrangel (to the north of Siberia) until some 5 ky and 4 ky respectively.
The last date (Vartanyan et al., 1993) is more than seven thousand years after the Younger Dryas, the last cold event of the
last glacial cycle. In the Mediterranean, at least five species of dwarf elephants of the genera Elephas (Palaeoloxodon) and
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A Genetic and Biological Perspective of the First Settlements of the Americas
Mammuthus became extinct over twenty islands. Most of these findings are not dated; the few existing dates are not quite
reliable but if correct they would point to the survival of elephants until as recently as 4 ky in the Aegean islands (Theodorou
et al., 2007 in Liscaljet 2012). A dwarf elephant is also present in Egyptian wall paintings of the eighteenth dynasty (near
3.5 ky), suggesting that the Egyptians coexisted with these now extinct animals in historical times (Masseti, 2001). Other
quite recent - possibly Holocene - losses in the Mediterranean islands included several species of dwarf hippopotamus (genus
Hippopotamus) from Crete, Sicily, Malta and Cyprus (Petronio, 1995).
Two of the most spectacular cases, however, were Madagascar and New Zealand -where the disappearance of the megafauna
was most recent. In Madagascar, a dozen species of spectacular giant lemurs became extinct between 2,000 and less than
400 years ago, when the last of them, the man-sized Megaladapis edwardsi, was seen by the French governor, Etienne de
Flacourt (Flacourt, 1658). Flacourt may also have seen the last of the half-ton elephant birds (Aepyornis maximus). When the
European colonizers arrived, huge elephant bird eggshells still littered the beaches of the island’s south and south-eastern
coasts, pointing to a very recent extinction indeed. Furthermore, Madagascar also lost pigmy hippos and giant fossas (Dewar,
1984). In New Zealand, there was an equally spectacular fauna of a dozen species of moas, large terrestrial birds up to three
metres tall, plus the gigantic Haast’s eagle Harpagornis moorei. All these huge animals went extinct as recently as between
900 and 500 years ago (Trotter and McCulloch, 1984).
Unlike any other extinction wave in the geological past, this one appeared to be size-selective: only the largest animals were
extinct (Lyons et al., 2004). The Quaternary extinctions deprived the planet of most of its large animals, but had little effect
on the small ones.
The aim of this article is to briefly discuss how the late Quaternary extinctions relate to human dispersal across the planet, how
this affects our views of human history, and the role of American sites in conserving this memory.
Pitfalls of the climatic explanations for the late Quaternary extinctions
Ever since Wallace, there has been much controversy on what caused the Quaternary extinctions, a dramatic revolution in the
history of life so close to our time. The hypothesis that the demise of the big animals had been caused by climate changes was
a favourite for near a century after Wallace (Grayson, 1984); it is still popular and has been proposed in many regional studies
to explain the disappearance of Pleistocene faunas (review in Koch and Barnovsky, 2006). From a global perspective, however,
the climatic hypothesis fails to explain too many of the extinction patterns; six of its main pitfalls are briefly pointed out below.
Firstly, climatic hypotheses do not explain the regional asynchrony of the extinctions. Climatic events linked to the last glacial
cycles were planetary events that affected the whole planet at the same time, for example, cold peaks were also linked to
dry periods in tropical regions. Nevertheless, as we saw, the extinctions took place at completely different times in different
places around the world.
Secondly, the chronology of the last glacial cycle does not explain the timing of extinctions in several parts of the world. For
instance, in Australia the extinctions occurred mostly within a relatively mild period about twenty thousand years before the
Last Glacial Maximum that lasted from about 23 to 18 ky. On the other extreme of the time scale, the insular extinctions all
occurred several thousand years after the end of the Younger Dryas, the last cold event of the glacial cycle, at about 13 to
11.5 ky (Burney and Flannery, 2005).
Thirdly, in the Pleistocene there were at least 31 other glaciations before the last, without any wave of extinctions associated
to them (Cione et al., 2003; Barnosky et al., 2004; Johnson, 2006). The recent suggestion by Wroe et al. (2013) that the last
glacial cycle was notably more intense than the previous ones, at least in the Sahul, is not convincing because intensification
can be observed only in the warm (interglacial) phases that which are not implied in the extinctions in the climatic hypothesis.
On the other hand, there is little difference where it matters, that is, among the cold extremes of the last cycles (see Wroe et
al.s own figure 5).
Fourthly, climatic hypotheses cannot explain why the extinctions on islands all over the world, independent of their latitudes
and longitudes, were systematically more recent than in the continents - all of them in the Holocene, a period of relatively
stable climate.
Fifthly, climatic hypothesis would predict many extinctions among plants that are usually more affected by climate changes
than animals, but this prediction conspicuously fails: there was no wave of floristic extinctions in the Quaternary.
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A Genetic and Biological Perspective of the First Settlements of the Americas
Sixthly, the climatic hypothesis predicts that small warm-blooded animals would be more affected than large ones, because
the former have higher surface/volume ratios and thus less efficient homeothermy (Thompson, 1917). But what happened
was exactly the opposite: the large animals were the ones that disappeared.
The Quaternary extinctions are a global phenomenon that begs for a global explanation. Arguably, a major reason why the
subject still seems puzzling is that many studies have tried to explain the extinctions using regional approaches, thus losing
sight of the immense explanatory power of a comparative, global analysis. The pieces of the puzzle indeed immediately fit
together when we observe the close similarity between the dates of human arrival and the extinctions in each land mass. A
quick review of modern man’s spread across the globe may be useful here. I make no claim to present a detailed appraisal of
the complex process of colonization of our planet; rather, I intend to provide only a brief sketch of the main events.
Man’s dispersal across the planet
Modern humans originated in eastern Africa and had most of their existence restricted to a relatively small part of the world.
The oldest dates for our species are at least 160 ky and the oldest ones out of Africa - in the Middle East - are about 120 ky.
From the Middle East, modern humans dispersed to Tropical Asia and remained restricted to these regions until about 50 ky
(Stringer and Andrew, 2005).
From there, the first new land mass to be colonized was Australia, reached by humans at about 50 ky. As the colonizers came
from the north, the last part of Australia to be reached was the southern tip, where Tasmania lies. At the time, Tasmania was
connected to Australia, New Guinea and smaller islands forming a single landmass, Sahul.
The expansion of humans towards the temperate parts of Eurasia started at a roughly similar time, at over 40 ky, but this was
a much longer process as humans seemed to have reached the cold areas of the gigantic Eurasian continent, such as northern
Siberia, by less than 20 ky. It was the first time humans had to colonize a continent against a temperature gradient - from
warm to cold - and possibly this factor helps to explain why it took so long (Araujo, 2013).
However, once reaching the eastern tip of Eurasia, humans had free access to the Americas, as Eurasia and North America were
then connected through Beringia. North America was reached around 15 ky or a little before this. At the time, northern North
America was still covered by a thick ice cover, because the deglaciation following had still not finished. Progression through
this part of the continent seems to have been slow, but at about 13.5 ky is the first evidence that humans had reached North
America’s central plains.
Within the Americas, and especially in South America, the colonization frontier seems to have expanded at quite different
speeds in different directions. The oldest reliable archaeological dates in South America are from Monte Verde, near Puerto
Monte, in southern Chile, at over 14 ky. This striking date seems consistent with the hypothesis of a faster colonization route
along South America’s Pacific coast. For pre-technological people, the Andes were surely an imposing geographical barrier
and this may have conditioned a ‘fast’ (lasting a few hundred years) movement southwards through the narrow land strip
between the Pacific and the gigantic mountain chain. To the east of the Andes, with a more complex geography and dense
forests, human expansion seems to have been much slower than on the west coast.
The last places to be reached by humans were the islands, all around the world, for the obvious reason that reaching them only
became possible after the invention of efficient watercrafts, capable of traversing extensions of saltwater (with the exception
of land-bridge islands such as Tasmania, which were connected to the continents at times of low sea levels during the late
Pleistocene). The first important oceanic islands in the Americas that were reached by humans were the Caribbean islands, at
nearly 6 ky. The Mediterranean islands seem to have been reached slightly later, and Wrangel, the home to the last mammoths,
was occupied at about 4 ky. Madagascar, one of the world’s largest islands, was first reached by humans only about 2,300
years ago and surprisingly, the first colonizers came from south Asia rather than Africa. Finally, New Zealand was the world’s
last large landmass to be reached by humans, just about 900 years ago.
A quantitative, global analysis making use of improved dating
The striking similarity between these dates of human arrival and the dates of the extinctions at the different landmasses across
the world has been pointed out by many authors, including Martin (1984, 2005), Fernandez (2000), Lyons et al. (2004),
Burney and Flannery (2005), Johnson (2006) and Gillespie (2008). However, Bernardo Araujo (2013) provided the first global
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A Genetic and Biological Perspective of the First Settlements of the Americas
Fig ure 1 . The ti mi ng of hu man a rri va l a nd me gafau nal
gen era e xtinc tio n acr oss t he wo rld i n t he la te Pl eis to cen e and
Hol ocene , as ana ly zed by B ernar do Ar auj o (20 13) . The d isp layed
qua ntita tive axe s sho w the last kn ow n d ates of oc cur re nce
at each landm ass ( cal ibrat ed da tes , in years befo re pa st) of
meg afaun al ge ner a (re d dot s). G rey s pea rs po int t o t he ti me of
fir st kn own d ate s of human pres enc e at each landm ass . The larg e
gre y arr ows show th e mai n rou tes of h uma n dis persa l acr oss t he
glo be. © Bern ard o Ara ujo.
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A Genetic and Biological Perspective of the First Settlements of the Americas
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A Genetic and Biological Perspective of the First Settlements of the Americas
quantitative analysis of the relations between the timing of the megafaunal extinctions, climatic changes and human arrivals
to each of the world’s land masses, taking advantage of the massive recent progress in dating methods. Using inaccurate time
frames in quantitative analyses (for example, as in Prescott et al., 2012, for human arrival), it would be difficult to disentangle
the effects of different putative causes for the extinctions (Lima-Ribeiro et al., 2012).
In Araujo’s (2013) analysis, the intensity of climatic variation in the late Pleistocene and Holocene was assessed through the North
Greenland Ice Core Project data on oxygen isotopic composition on ice cores (Andersen et al., 2004). This database comprises ƣ18O
data (a temperature proxy) for the last 122 ky. Megafaunal dates and human dates were obtained from literature for nineteen regions
of the world where these events took place at markedly different times. The regions considered in the analysis were Australia, the
Caribbean, Japan, Madagascar, New Zealand, Tasmania, Wrangel Island, Beringia (north-westernmost North America and north-
easternmost Eurasia), central North America, eastern North America, western North America, northern Europe, southern Europe,
west Siberia, central Russia, northern South America, central South America, Patagonia and the Andean region (the Andes and
South America’s western coast). Only calibrated dates, regarded as reliable according to the Mead-Melzer modified scale (Barnosky
and Lindsey, 2010) were used. The database included 2,088 megafaunal dates (for 67 genera) and 762 archaeological dates.
Araujo’s (2013) main results are summarised in Figure 1. The dates shown are the last known occurrences of each megafaunal
genus and the first known human presence in each landmass. For sampling reasons, the last known date of a fossil genus tends
to overestimate how long ago the extinction occurred and the first known human date tends to underestimate how old is the
arrival (Signor and Lipps, 1982; Buck and Bard, 2007). Taking this in account, the clustering of the extinctions dates around the
times of human arrival can be clearly seen. Comparing the two variables (human arrival and climate variation), the extinction
times were nearer than expected by chance only to the time of human arrival in 76 cases (each case being the extinction of
a genus in one of the 19 regions). On the other hand, the time of the extinction was closer than expected by chance only to
the times of intense climate change in just 2 cases (a value close to the expected by a random placement of the extinctions
along time). Extinctions were closer than expected by chance to both human arrival and times of intense climatic variation in
8 cases, and to neither of these variables in 40 cases, out of a total of 126 cases. These findings provide clear support for the
hypothesis that anthropogenic impacts, rather than climate variation, were the main driver of the late Quaternary extinctions.
The eight cases where the extinctions were close in time both to human arrival and to climatic variation were mostly in the
Americas. In these continents, human arrival approximately coincided in time with the events linked to the Younger Dryas,
the last cold phase of the last glacial cycle. This finding is consistent with the view that climate changes actually played a role
in the late Quaternary extinctions. But what role was this? Several authors have interpreted this kind of result as a ‘synergy’,
for the Americas (Barnosky et al., 2004; Lima-Ribeiro and Diniz Filho, 2013) and elsewhere (for example, Nogués-Bravo et al.,
2008, Lorenzen et al., 2011). However, this terminology can be misleading, by seeming to attribute similar roles to both factors:
climate and humans. Synergy refers to an interaction between two or more factors that, when acting together, produce an
effect that is larger than or different from the sum of their individual effects. However, the global approach provides ‘natural
experiments’ in time and space that allow us to separate the factors: the megafauna survived through many other intense
glacial cycles throughout the whole Pleistocene and it also persisted well into the Holocene in islands not yet reached by
people. Both lines of evidence show that if it were not for the anthropogenic impacts, it is unlikely that the megafauna would
have become extinct. Thus, climate changes seemed to have acted mostly as intensifiers, when they coincided with humans’
arrival in a given landmass. In the context of logic, one could say that human arrival was the necessary cause of the extinctions
- without it, the megafauna would not have disappeared - while climate change was just a contributory cause - by itself it
would have little effect, but once present it played a role as well.
The associational critique and why it does not refute the anthropogenic hypothesis
Within archaeological contexts, a question often raised regarding the anthropogenic hypothesis is, if it is correct, then why
there are relatively few sites showing associations between humans and megafauna? This has been called the ‘associational
critique’ (Meltzer, 1986). Is this pattern incompatible with the hypothesis of anthropogenic impacts?
To answer this question, lets suppose just for a moment, that the anthropogenic hypothesis is correct. If so, using the
mammoths as an example, in which proportion of mammoth sites would we expect to find associated archaeological remains?
First, we must keep in mind that the time of coexistence of mammoth and humans, until mammoths became extinct, was a
tiny part of the geological time range of mammoth records (Lister and Bahn, 2009). But this is still not the point. About twenty
centuries in North America was the time of coexistence of mammoths and humans in the same continent, but in any one
locality the coexistence was much shorter still. Thus, one would have to be very lucky indeed to find an archaeological site from
precisely the few decades or so when mammoths and humans coexisted in that given locality. In any other case, he (she) would
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A Genetic and Biological Perspective of the First Settlements of the Americas
find only mammoths without humans (before coexistence) or humans without mammoths (after coexistence). The expected
pattern would be that a small fraction of megafaunal sites would have archaeological remains, and that is what is found.
Recently, Surovell and Grund (2012) presented another, complementary, refutation of the associational critique. They compared
the relative temporal depths of coexistence (time spans between human arrival and megafaunal extinction, in proportion of
the whole length of the archaeological record) among Australia, North America and New Zealand. Furthermore, they modelled
taphonomic biases and also human demography to estimate how long it would take for humans to become abundant in a
given landmass after their first arrival. Their results show that kill sites would be expected to be very common in New Zealand,
to have intermediate frequency in North America and to be extremely rare or absent in Australia. This order of frequency of
kill sites again is similar to the patterns we observe in the real world.
A modern view of the anthropogenic hypothesis
It is interesting to think about the process that resulted in these patterns. The hypothesis that the late Quaternary extinctions were
caused by anthropogenic impacts has been considerably improved, in light of more recent knowledge, since it was put forward
by Paul Martin (1967, 1984). A modern view of the anthropogenic extinctions as part of our early history would be as follows.
Except where megafauna co-evolved with early hominids (in Africa and south Asia, where there were few extinctions, and northern
Eurasia, where they were quite spread across time), there is no reason to expect that big animals would have evolved instincts to avoid
man. This is the phenomenon of ‘island naivety’, well known from many historical examples (Diamond, 1984). In the most plausible
scenario, big beasts in the newly-colonized lands were quite unaware that humans could be dangerous hunters - to their downfall.
Putting the extinctions in the perspective of the time in which they happened, at least in three aspects, helps us understand
that crucial period of human history. First, there was neither agriculture nor livestock - probably not a coincidence, as they were
invented only after the megafauna disappeared. Thus, consumption of meat from large animals was probably indispensable
for feeding a growing human population and if humans had any access to this resource - and technologically they had - one
would hardly expect that they would not have used it. Second, once a large beast was killed, there was no efficient way to
conserve vast amounts of meat. There were some valiant attempts such as underwater meat caching, first discovered in the
Heisler site in Michigan (Fisher, 1995), and possibly the use of salt as well - but this was hardly enough. Therefore, a highly
efficient use of megafaunal meat is unlikely. Third, people living at the time of our species’ dispersal did not even know that
the world had an end. They went on finding virgin hunting fields, until one day - in the Americas, probably the day when the
southernmost tip of Tierra del Fuego was reached - there were no more. Given these three points, if we put ourselves in the
shoes of a Clovis or any other people of the time, how could we expect them to use the megafauna in a cautious, sparing way?
What for, not knowing that the world had an end? Such restraint may make sense from our perspective, but not from theirs.
Although in its original modern formulation (for example, Martin, 1967, 1973), the hypothesis of anthropogenic impacts puts
much emphasis on hunting as the main extinction mechanism (hence its common label the ‘overkill hypothesis’), more modern
views have highlighted a diversity of anthropic impacts, all of which must have affected megafauna to different extents. Among
these factors are introduced diseases (MacPhee and Marx, 1997) and fire. The later process is well documented, especially
in Australia (Flannery, 1994; Johnson, 2006; Rule et al., 2012) and in North America (Flannery, 2001; Kerr, 2003; Gill et al.,
2009). Increased frequency of fire actually seems to have been a consequence of the decline of plant-eating beasts, but it
may have further affected the megafauna through habitat changes. Besides, some of the extinctions must have been caused
indirectly through cascading ecological effects, especially the demise of large carnivores and scavengers. It is unlikely that the
Clovis extinguished sabertooth tigers through hunting them for meat; there must have been easier sources of protein around.
However, after all their big prey species were gone, sabertooths were doomed as well. The same must have applied to the
huge North American teratorn birds and other large scavengers that depended on megafauna. The California condor’s habit
of scavenging on stranded marine mammals must have emerged as a much-needed ecological niche shift after its usual food
sources had all but disappeared.
Another point is that it was not a ‘quick’ extinction at all, as had been proposed by Martin’s ‘blitzkrieg’ version of his hypothesis
(Martin, 1973). The extinctions were a long historical process that lasted several thousand years in each continent, with a
few stragglers left even after that, as expected - the last on the islands. A comment here about the time scale involved in the
events here described could be useful. Palaeontologists work with so-called ‘geological time’, which comprises very long time
scales, often in the order of millions or billions of years. Besides, the farther away we look to a given time span in the past, the
shorter it seems. There seems to be little difference between 40,500 to 40,000 years ago, but from Columbus to us it seems a
long time. This trap of our perception and Martins unfortunate expression help to explain many misunderstandings about the
anthropogenic hypothesis. ‘Blitzkrieg’ means lightning war, but it was not a war at all, nor was it quick. As far as we know, the
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A Genetic and Biological Perspective of the First Settlements of the Americas
animals were hunted not because of our species’ urge for killing, but because they provided valuable resources for which there
were no substitutes at the time. It was a very long process that took several millennia in continents and centuries in large islands.
The Quaternary extinctions do not imply that humans were terrible hunters. Big, slow-breeding animals are the most vulnerable
to hunting. The species that became extinct had low fecundity, usually approximately less than one young per female per year
(Johnson, 2002). Wallace’s first impression - that the extinctions affected preferentially large animals - is an artefact of the
negative correlation in nature between body size and breeding rates. Big animals might not have even been a major component
of the diet and even so, they would be expected to become extinct. For example, Johnson (2006) estimated that a group of ten
Australian aborigines needed to kill only about two diprotodonts per year to extinguish a population of these huge marsupials.
The point with the late Quaternary extinctions was not the intensity of the hunting - it was the fragility of the hunted.
Altogether, from an ecologist’s point of view, it is not surprising that humans’ arrival in the new continents would have
extinguished megafauna - it would be surprising if it had not.
The conservation of the memory of the human-megafauna
interactions and the south-central American sites
In light of the evidence now available, without the anthropogenic impacts following human dispersal, nobody would be
talking of ‘Quaternary extinctions’ today. It remains quite plausible that anthropogenic impacts were magnified by climatic
events in some places, but climate change does not seem to be a major cause. However, regardless of whether anthropogenic
impacts were the main cause or ‘just’ one of the major causes, what we know today is more than enough to show that
these extinctions were one of the main events in the history of man’s interactions with natural environments, as our species
dispersed across the planet. Yet, perhaps because of the relative recency of Martin’s hypothesis, or perhaps because the extent
of humans’ role is still debated, these are undervalued pages of our history. The late Quaternary extinctions are a fascinating
subject that have seldom received the attention they deserve in education or in the media. The UNESCO World Heritage
Convention can have an important role in changing this scenario.
The Americas must play a special role in the effort for changing this scenario. After all, the Americas were the New World, not
just in the familiar, Eurocentric, cultural way - they were also the biological New World. The Americas were the last continents
to be reached by modern humans and therefore the continents where megafaunal extinctions as a whole were the most recent.
They therefore a have a crucial role in preserving the memory of our interactions with the extinct large animals. Herein, we
would like to tentatively propose three American sites which can be particularly valuable for this goal.
The first is Monte Verde in southern Chile. With its very old dating, Monte Verde is of course a key site in understanding human
dispersal into the continent. But there is more to it; Monte Verde also illustrates particularly well the use of megafauna by humans,
with a chunk of gomphothere meat found in an archaeological context. As Monte Verde is likely to reflect the oldest colonization of
the continent, this finding may well represent a snapshot of the early utilisation of the just discovered South American megafauna.
The second site is Serra da Capivara, within Serra da Capivara National Park, in Piauí state in north-eastern Brazil. This is also
a highly valuable site, not because of its unreliable claims to greater antiquity, but for its richness in rock paintings, many of
which represent extinct species and their interactions with the early settlers. To our knowledge, there is no other site which
represents so well human-megafauna interactions in Brazil. Besides, these paintings unfortunately are threatened by vandalism,
which increases the urgency of efforts to conserve them.
The third site proposed is Las Breas de San Felipe, in Cuba. It would be important to represent the last of the American extinct
megafauna and its interactions with humans. Hence, Cuba was home to the last surviving megafauna in the Americas. The
most recent records are from the sites of Las Breas de San Felipe and Solapa de Silex, with human artefacts associated to dwarf
giant ground sloths (genera Parocnus and Megalocnus respectively), dated to 5.7 and 4.7 ky respectively (reliable calibrated
dates). Las Breas de San Felipe is a tar pit, a kind of miniature Rancho Las Breas, the famous Californian tar pit site (Iturralde-
Vinent et al., 2000). Among Cuban sites, Las Breas de San Felipe is the richest, with easiest access and thus the one that
presents more potential to be nominated to the UNESCO World Heritage List, despite the slightly younger dating of Solapa
de Silex’s ground sloths (MacPhee et al., 2007).
Conserving evidence of human-megafauna interactions is important for us to learn from our history - about when our impacts
on nature were severe, when they were not, and what caused the different outcomes. It is also a useful reminder of the fragility
of nature – especially of the big animals – to our actions, in a time when so many people still deny the full magnitude of our
impacts on the natural systems. Now that we are extinguishing species at unprecedented rates, more than ever we need
to learn from the past. As Paulinho da Viola (1972) sang in ‘Dança da Solidão’, ‘Meu pai sempre me dizia / Meu filho tome
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A Genetic and Biological Perspective of the First Settlements of the Americas
cuidado / Quando eu penso no futuro / Não me esqueço do passado’ (‘My father always told me / My son, be careful / When
I think about the future / I don’t forget the past’).
Acknowledgements
First and foremost, I would like to thank Bernardo Araujo for all his help and discussions, for preparing Figure 1 and for allowing me
to use some of his unpublished data in this paper - which is also his. I am grateful to Nuria Sanz for her invitation to contribute to
this volume. These ideas owe much to discussions with José Alexandre Diniz-Filho, Matheus Lima-Ribeiro, Leonardo Ávila, Leopoldo
Soibelzon, Joaquín Hortal and the late Ibsen de Gusmão Câmara and Paul Martin, among others. I also thank the participants of the
‘The First Peopling of the Americas and the World Heritage Convention’ in Puebla, Mexico, especially Antonio Gilman, Chris Scarre,
Dennis O’Rourke, Ian Kuijt, James Chatters, Joaquín Arroyo-Cabrales and Tom Dillehay for their comments. Finally, I thank Anjelica
Young and Chantal Connaughton for their logistical support all along and for their careful review of the manuscript.
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Article
Full-text available
Here I offer the references to Figure 1 and Table 1 included in the article "The controversy space on Quaternary Megafaunal extinction published with my co-authors in Quaternary International
Chapter
Full-text available
Catastrophic hypotheses for mass extinctions are commonly criticized because many taxa gradually disappear from the fossil record prior to the extinction. Presumably, a geologically instantaneous catastrophe would not cause a reduction in diversity or a series of minor extinctions before the actual mass extinction. Two types of sampling effects, however, could cause taxa to appear to decline before their actual biotic extinction. The first of these is reduced sample size provided in the sedimentary record and the second, which we examine in greater detail, is artificial range truncation. The fossil record is discontinuous in time and the recorded ranges of species or of higher taxa can only extend to their last known occurrence in the fossil record. If the distribution of last occurrences is random with respect to actual biotic extinction, then apparent extinctions will begin well before a mass extinction and will gradually increase in frequency until the mass extinction event, thus giving the appearance of a gradual extinction. Other factors, such as regressions, can exacerbate the bias toward gradual disappearance of taxa from the fossil record. Hence, gradual extinction patterns prior to a mass extinction do not necessarily eliminate catastrophic extinction hypotheses. The recorded ranges of fossils, especially of uncommon taxa or taxa in habitats not represented by a continuous record, may be inadequate to test either gradual or catastrophic hypotheses.
Book
As recently as 11,000 years ago "near time" to geologists mammoths, mastodons, gomphotheres, ground sloths, giant armadillos, native camels and horses, the dire wolf, and many other large mammals roamed North America. In what has become one of science's greatest riddles, these large animals vanished in North and South America around the time humans arrived at the end of the last great ice age. Part paleontological adventure and part memoir, "Twilight of the Mammoths "presents in detail internationally renowned paleoecologist Paul Martin's widely discussed and debated "overkill" hypothesis to explain these mysterious megafauna extinctions. Taking us from Rampart Cave in the Grand Canyon, where he finds himself "chest deep in sloth dung," to other important fossil sites in Arizona and Chile, Martin's engaging book, written for a wide audience, uncovers our rich evolutionary legacy and shows why he has come to believe that the earliest Americans literally hunted these animals to death. As he discusses the discoveries that brought him to this hypothesis, Martin relates many colorful stories and gives a rich overview of the field of paleontology as well as his own fascinating career. He explores the ramifications of the overkill hypothesis for similar extinctions worldwide and examines other explanations for the extinctions, including climate change. Martin's visionary thinking about our missing megafauna offers inspiration and a challenge for today's conservation efforts as he speculates on what we might do to remedy this situation both in our thinking about what is "natural" and in the natural world itself."
Article
Many forms are represented only by isolated bone fragments, and comparatively little has been accomplished in the way of functional morphology and detailed description of the extinct beasts. Consequently interpretations are speculative and subject to revision. Observations imply that Australian fauna was (and still is) less able to withstand the impact of ecological change in the presence of man, even with the small numbers of men that were present 50 000-30 000 yr ago. -from Author
Article
Outlines the evidence concerning 1) geographical and chronological patterns of extinction and 2) the chronology and nature of human occupation of the island. Examines competing hypotheses about the relationships between these phenomena, and finally presents a revised model of the human role in Malagasy extinctions, where it is argued that ecological transformation resulted from the substitution of domestic bovids for the native terrestrial herbivores. Causal factors suggested are competition, habitat destruction and fragmentation, and adventitious hunting. -after Author
Article
Scrutinizes historic extinctions in a detailed attempt to fathom their meaning in the context of Late Pleistocene losses. Historic disappearances of modern birds and mammals can be laid to a variety of effects, some climatic, some cultural. Also considers 'the extinctions which did not occur'. -after Editor