The peopling of the planet should be considered a consequence of dispersal rather than intended migration. It should be imagined as a fluid process with people moving back and forth as opposed to a singular, directional colonization event (Figure 6.2).
The cause of the initial hominin dispersal out of Africa is not explicitly told in the fossil or archaeological records. An early hypothesis suggested that the invention of the Acheulean hand ax enabled H. erectus to inhabit new territories. It could, in a sense, afford to venture out with its new technology. The idea is based on the association of the ubiquitous hand ax with H. erectus and the presence of hand axes outside of Africa, The assumptions behind this idea are too transparent for most scientists today considering elephants dispersed from Africa without the benefit of hand axes.
Something more biological as opposed to cultural was probably the cause of H. erectus dispersal. A predatory species cannot survive in large groups. Since it sits atop the trophic pyramid, a predator cannot outnumber its prey or it will starve to death. Pat Shipman and Alan Walker proposed that in order for H. erectus to make the carnivore transition, it probably either reduced its population size or increased its geographic range and then also increased its body size. The fossil record indicates the latter two clearly occurred after 1.8 Mya, just about when evidence for meat-eating becomes prevalent. The Old World dispersal of H. erec-tus was probably a consequence of becoming a predator and simply following the herds.
The cause behind the dispersal of modern humans could have been the result of similar pressures. Predatory issues were most certainly an issue for early humans, however by the emergence of H. sapiens we have the first clear-cut evidence of fire, shelter, a higher degree of sociality, a brain size increase, enhancement of technological skill. The evolution of technology and adaptability probably facilitated the expansion of humans into more marginal and previously inaccessible environments compared to H. erectus. There is also abundant evidence that modern humans in various regions were incorporating a variety of prey items like tortoises, hares, ground birds, and shellfish into their diets, so not all populations were necessarily following herds.
Sites along the world's coastlines suggest that early human populations were successful at living on shores and this lifestyle probably facilitated dispersal especially by the time boats were ubiquitously used. The origins of boats and seafaring crafts are unclear at present but it is generally safe to assume that the first Australians between 60 and 50 Kya had such inventions. The colonization of Australia is intriguing because even at low sea levels there was never a land connection between the Sunda continental shelf of Northwest Indonesia and the Sahul continental shelf, which contains the Australian landmass as well as Papua New Guinea and Tasmania. Australia is an unusual case because the continent may not have been visible from any Indonesian islands, even at times when sea levels were lowest during glacial periods when water is tied up at the frozen poles. Although bird flight patterns could have indicated land beyond the horizon, the distance is considered too far for people to simply island hop the way H. erectus (and maybe Homo floresiensis) did throughout Indonesia. To get to Australia, it is assumed that people would have had to build boats or crude rafts and endeavored out onto the open sea to explore. Getting to Australia probably required technical seafaring skills as well as the ability to plan, work as a group, and probably talk to one another. Once on the big island, there is evidence that they moved to the southern part rapidly. By 23 Kya at Willandra Lakes in southeastern Australia, early inhabitants left behind the largest collection of Pleistocene human footprints in the world. Fossil skulls and skeletons from the earliest sites like Kow Swamp in Northern Victoria and Lake Mungo show that only modern humans were able to reach Australia.
The first Americans were also probably seafaring people. They came from northeast Asia and were not just good at the marine lifestyle but that they also adapted to an arctic one. Genetic analyses estimate that humans first arrived in the Americas as early as 30 Kya, but more likely between 25 and 15 Kya during the height of the last glacial period. The most probable route was across the Bering Strait from Siberia to Alaska which, when sea levels were lower, would have been largely closed and some Aleutian islands would have been linked, making the journey (although not necessarily an intentional one) more terrestrially anchored and therefore less treacherous.
Jon Erlandson suggests that the first Americans could have grazed their way from Asia to the America on an "ancient kelp highway." According to the "coastal migration theory," they may have lived in small maritime populations that boated from island to island, or shore to shore, hunting the sea creatures that lived in kelp forests. At present there is a nearly continuous kelp highway from Japan and Siberia, across Bering Strait to Alaska, and down the California coastline. And in kelp forests live, for instance, seals, sea otters, fish, and sea urchins. Some of the earliest archaeological sites in the Americas are found near productive kelp forests. Daisy Cave in the Channel Islands off southern California, dated to nearly 10 Kya, has preserved evidence that humans used kelp resources there.
The Clovis culture, named for its discovery in Clovis, New Mexico, is known for its elegantly distinctive spear points and dates to 11 Kya at the end of the last Ice Age. The Clovis people were long given the title of the earliest humans to arrive in America, but the geography of most sites is skewed to the eastern side of North America, while earlier and contemporaneous pre-Clovis sites of human occupation have been found in the west and in South America. Duktai in Alaska is about
12 Kya, and others like the Meadowcroft rock shelter in Pennsylvania, Topper in South Carolina, Taima-Taima in Venezuela, Pedra Furada in Brazil, and Monte Verde in Chile are as old and could be even older. Meadowcroft in particular may be as old as 20 Kya. By 12.5 Kya in Monte Verde, Chile, archaeologists found the earliest permanent settlement in the New World with the remains of a shelter that would have been large enough to house upwards of thirty people. Just like the dispersal out of Africa, there were likely multiple influxes of human populations into the New World and the continued discovery of archaeological sites as well as the deeper analysis of genetics of living human populations will help recount the true history of the first Americans.
Did NEANDERTHaLs Discover AMERica?
While screening the dirt and rocks from a creek bed looking for evidence of the Santee Canal built in the 1790s (30 miles west of Charleston, South Carolina), underwater archaeologist Mark Newell recovered a crude stone tool. It was a remarkable find considering the tool is a type that was made by Neanderthals in Europe. Plus, the raw material was obviously foreign. Microscopic invertebrate fossils in the rock confirmed that it originated in France. In fact, the unique geochemical composition of the stone pointed to its source in a particular rock formation in the Bordeaux region. Could Neanderthals have floated across the Atlantic in the Stone Age to discover North America?
It would be tempting to conclude Neanderthals were the first Americans, but with further evaluation a simple explanation is clear. In relatively recent history, European rivers that hold prehistoric artifacts were mined for ballast, and then European ships dumped their ballast in American ports when taking on cargo for the voyage back home. These dumps were then mined for ballast that was recycled in local American watercrafts, like those that traveled in the Santee canal. --
WILL WE EVOLVE OR WILL WE GO EXTINCT?
There are tales born from misunderstanding evolution that humans will eventually lose their diminutive fifth toe or their rupturing appendices. Men are often projected to lose their milkless nipples and women are forecasted to develop extra-large or even additional breasts to accommodate exaggerated sexual preferences. It is even commonly assumed that the world's population will eventually share a single skin tone, even though we know from both life experience and genetics that skin color inheritance is not as simple as mixing each parent's pigments together.
These speculations lead naturally to the question: Is natural selection working on us anymore? From examples like lactose tolerance, we know that evolution has brought about major changes to human populations within just the last several thousand years. Resistance to HIV infection and malaria is higher in some populations than others. Clearly human cultural adaptations and innovations are not preventing us from developing some biological ones. But if natural selection is currently acting in human populations, does that mean speciation is on the horizon? And can we dare to ask such a thing without flirting with racism?
With modern modes of global travel and with decreasing cultural barriers between populations (languages are going extinct all over the world), gene flow is most likely going to prevent any new species from branching off of humans. With 6 billion of us on the planet, it seems unlikely that any human population could become so reproductively isolated that it splits off into a separate species, but such a scenario is not outside the limits of reality especially in the wake of a hypothetical worldwide catastrophe like the dinosaurs experienced. Fortunately NASA is constructing plans for diverting an asteroid like the one that caused the dinosaur extinction should one threaten Earth again.
Avoidable or unavoidable catastrophes aside, can we use medicine to keep us from going extinct? Medicine along with agricultural technology will probably help keep H. sapiens from a predictable demise, but they cannot help us evolve into more extinction-resistant forms. Unless medical changes are made to the gametes and inherited in future generations, all the best organ regrowth and antiaging treatments will mean next to nothing in evolutionary terms. But natural selection could still work on fertility or fetal survival and sexual selection can shift things as well. There is evidence that a number of our genes involved in lactose tolerance, brain development, skin pigmentation, reproductive organ development, metabolism, and disease resistance are undergoing strong selection right now which will most likely lead to changes at least at the population level in the future. As a species, we will continue to evolve into the future, but how much?
We could evolve into forms so different from our current ones that future biologists and paleontologists would call us a different species. In that sense, H. sapiens as we know it now would be extinct. We may not have much longer to evolve, however, because the next big faunal turnover, or cyclical extinction event, is predicted to occur at around AD 500,000. On average, mammal species last about 2.5 million years, which is the duration between turnover events. It is probably no coincidence that drastic evolution in the hominin lineage corresponds to the last turnover event around 2 Mya. These extinctions are spaced by intervals of climate change (affecting temperatures, precipitation, habitats, and food sources) that are controlled by astronomical variations like changes in the Earth's orbit and its wobbling on its axis (Milankovitch cycles). 500,000 years is plenty of time to evolve into something new, after all, 500 Kya there were only moderately intelligent Archaics and H. erectus on Earth and look where hominins stand today.
But while we thrive in a population of 6 billion, our closest relatives are on the brink of extinction. The World Wildlife Fund (www.worldwildlife.org) reports that great apes are all endangered. There are only about 10,000 bonobos left in the wild, down from over 100,000 just twenty years ago. They are being squeezed out of preferred habitats by humans and are also being killed for bush meat to be served at expensive restaurants. Orangutans are stuck on islands that are becoming increasingly deforested. Their numbers are dwindling at 50,000. Jane Goodall reported on her Web site (www.janegoodall.org) that there were only 150,000 chimpanzees left in 2004, but that there should be at least be a million today. Less than 1,000 mountain gorillas have survived to the present day with no help from poachers and deadly Ebola outbreaks.
Witnessing the inevitable wild extinction of our closest relatives reminds us just how adaptable we are, but just because we normally tell one single evolutionary tale (our own) does not mean that all of evolution culminated in the result of humans. Although, for many people, being human means being elevated from the fishes and the rest of the animal kingdom, simply deciding that humans are the king of the planet does not mean we actually are. Granted humans cover nearly the entire dry surface of the world and some of the wet surface as well, but there is only one species of upright apes alive today after a mere 6 million years of evolution, yet there are over 350,000 species (and counting) of beetles which have been evolving for over 260 million years. In that regard, beetles are the king of the planet having successfully radiated and propagated far more than humans ever have and ever will.
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