Humanity’s shadow: The spread of man across the Earth

It all started with a woman. Or at least, we can trace it back to one. The origin of the species Homo sapiens, a likely descendant of Homo erectus can be linked to the DNA of a single female specimen scientists called ‘Eve.’

Now before everyone starts preparing for the rapture we must be clear that Eve was not our earliest ancestor, nor was she the first woman. Rather, she was an early modern human from a time when modern humans were rather rare. The important thing to note is: Eve, and everyone else at that time, originated in Africa.

Figure 1: Homo erectus

Figure 2: Homo sapiens

The date for the advent of Homo sapiens is around 200 000 years ago. Prior to this, Homo erectus, the first hominid to tame fire, is thought to have migrated from Africa to Arabia, Asia and Europe some 800 000 years ago. Homo sapiens soon outstripped its predecessor however, moving out of Africa around 125 000 years ago. Between 85000-75000 years ago, humans spread from Arabia to India, Borneo and South China, hugging the coast all the while. By 60 000 years ago, Australia was no longer virgin territory, humans having crossed there from South-east Asia. Finally, a changing climate 50 000 years ago was the catalyst which allowed humans to migrate up into Europe. Central and North-east Asia were next for our intrepid ancestors, and the colonisation of Siberia around 40 000 years ago preceded expansion into the Americas.

The colonisation of the New World was remarkable in that the spread of humans was so rapid. Indeed, we could say it was plague-y. Human predation is listed as a primary reason for the disappearance of 35 genera of large mammals including ground sloths, giant tapirs, large predators and mammoths. Plague or not, humanity managed to expand over the course of only 10 000 years from North America down into South America. There is evidence that the expansion to the Americas may even have involved watercraft, with humans expanding along the coast then into the interior of these continents. Regardless of how it was achieved, by 12500 years ago, our distribution was almost global. An excellent interactive map of this human migration is available here.

Figure 3: The now extinct Giant Ground Sloth

In a relatively short time (in evolutionary terms), humans learnt to co-operate, to live in communities, communicate ideas and even to sail. We colonised islands and icy places, increasingly using our technology and great ingenuity to shape the world to our own ideal.

References

BROWN, J.H. & LOMOLINO, M.V.1998. Biogeography. 2^nd Edition. Sinauer Associates Inc., Massachusetts.

GUGLIOTTA, G. 2008. The Great Human Migration. http://www.smithsonianmag.com/history-archaeology/human-migration.html?c=y&page=5 (Accessed 25/04/2013).

GRAYSON, D.K. 2001. The Archaeological Record of Human Impacts on Animal Populations. Journal of World Prehistory 15(1): 1-68.

The ravages of ice: How glaciation affects biotas

Imagine a sheet of ice creeping inexorably across the landscape over millions of years, moving downwards from the poles. Regions once mild in climate become part of an ever expanding frozen wasteland. This terrible, slow process of advancing ice is called glaciation. In the past, glaciation events have shaped our planet. Individual glaciers can carve through rock and flatten landscapes. The raw power of ice has shaped the very nature of life on Earth.

The general cause of glaciation events is a change in Earth’s mean temperature and an increase in snowfall at higher latitudes. The underlying causes of such temperature shifts however vary. Factors which can cause the Earth’s mean temperature to drop by a few degrees Celsius include a large-scale increase in volcanism and meteor collisions. In both scenarios, less solar energy reaches Earth’s surface due to more dust in the atmosphere, and the temperature drops as a result. Glaciation is not a randomly occurring process however and the commonly accepted  theory to explain patterns of glaciation is that of Milankovitch cycles.  Milankovitch was a Serbian scientist who observed that not only does the angle of Earth's tilt change, but that our planet 'wobbles' on its axis. He also noted that the Earth's distance from the Sun varies as the orbital path our planet follows changes in shape. The force behind the global temperature changes during glaciation is thus once more a change in how solar radiation reaches Earth. As a result of such fluctuations, climatic conditions on Earth are dissimilar during glacial and inter-glacial periods.

Figure 1: Variations in Earth's orbit associated with periods of glaciation.

Naturally, such changes affect Earth’s biota. Regions once suitable for particular species become less so and the range of species may contract or the species may suffer extinction. Other species may have the opportunity to expand their ranges. Species ranges may also simply shift in latitudinal terms. Entire biomes may move polewards or towards the equator. Another possibility is that a species will be able to adapt to the new conditions within its current distribution. The nature of the response by individual species depends on the characteristics of those species. In order to survive a period of glaciation, a species must both adapt to changes in its physical circumstance such as altered conditions of climate and geography and be intrinsically suited for the new world in which it finds itself or able to disperse to a suitable area.  Over time, natural selection may result in a change in the morphology of a species. Bigger individuals for example may better survive an icy world due to their superior heat retention and these genes may be passed along preferentially.

Besides changing the distribution of species, glaciation may cause entirely new species to form. One of the driving forces of speciation is geographic isolation. When a glaciation event occurs, species may retreat to refugia – isolated pockets of suitable habitat – and within these, entirely new species may form over tens of thousands to millions of years. When the ice retreats, these species can once more spread, expanding their ranges into new suitable territory.

Figure 2: Species may escape unfavourable climatic conditions by retreating into refugia

Due to the extinction of some species and the adaptation or formation of others, periods of glaciation change the structure of communities and eventually the biotas of entire regions. Further changes take place when the ice recedes. New species spreading into new territory may compete as they expand their ranges and further extinctions may occur. Though some communities may survive unchanged during glaciation, entirely novel ones will also form. Thus as the tide of ice expands and retracts over geological time, it leaves in its wake a changed world.

Works Cited:

BROWN, J.H. & LOMOLINO, M.V. 1998. Biogeography. 2nd Edition. Sinauer Associates Inc., Massachussets.

PARMESAN, C., ROOT, T.L. & WILLIG, M.R. 2000. Impacts of Extreme Weather and Climate on Terrestrial Biotas. Bulletin of the American Meteorological Society 81(3): 443-449. Available online at: http://journals.ametsoc.org/doi/pdf/10.1175/1520-0477(2000)081%3C0443%3AIOEWAC%3E2.3.CO%3B2 (Accessed 11/04/2013).

RAND, A.L. 1948. Glaciation, an Isolating Factor in Speciation. Evolution 2(4): 314-321. Available online at: http://www.jstor.org/stable/2405522 (Accessed 11/04/2013).

RITTER, M. E. The Physical Environment: An Introduction to Physical Geography. 2006. Available online at: http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/title_page.html

(Accessed 11/04/2013).

SCHIEBER, J. Milankovitch Cycles and Glaciation.  http://www.indiana.edu/~geol105/images/gaia_chapter_4/milankovitch.htm 

(Accessed 12/04/2013).

Nature's Laboratories: How Islands Shape Evolution

The idea of islands as laboratories where natural selection expresses its stranger side is not difficult to justify. Many species on islands are unique, and uniquely different from their mainland counter-parts. Dwarf elephants, giant carnivorous birds, the world’s largest stick insect (and largest insect to boot) – all of these are (or were) the inhabitants of islands. The biotas of islands often consist of multiple endemic species , and whilst the term island conjures up images of tropical paradise, for biological purposes an island can be taken to mean anything from an isolated mountain-top, to a patch of forest in otherwise monotonous grassland.

Figure 1: Elephant bird (Aepyornis maximus)

The capability of mainland species’ to reach islands is of importance in how well the biotas in these restricted areas end up reflecting the composition of mainland biotas.  As such, two broad divisions can be made: harmonic and disharmonic biotas. Harmonic biotas are those in which the assemblages of organisms present are similar to that found on the mainland in that whilst fewer species occur, proportionately the ecological groupings or taxa are the same. Disharmonic biotas result due to a difference in dispersal ability of organisms, as well as factors such as competition. Essentially, those organisms which are better at reaching islands (such as flying animals or wind-borne seeds) may be disproportionately represented. Other taxa from the mainland may never make it to the island at all, and, naturally the more isolated the island the fewer species are expected to occur there. The larger an island’s size, the more species may reasonably be expected to survive there. These are all common sense rules of island biogeography .

Once a population becomes established in its new island home, the different selective pressures it faces may result in some very strange changes in evolutionary development. Some of these evolutionary trends are common to island organisms. Reduced dispersal ability is one such development. Now it may seem counter-intuitive that organisms on islands should lack the ability to leave, but loss of long distance mobility has advantages. In insects, specimens which restrict their movements to a home range area are more likely to avoid death at sea on an oceanic island, thus they will survive and breed. Flightless birds sacrifice mobility for lower energy investment. They no longer have to grow strong flight muscles, as predators are often missing from island areas. Coupled with the fact that resources may be scarce, it makes sense to avoid unnecessary commitment of energy to flight.

Flightlessness can also be rooted in another island phenomenon: Gigantism. On islands, organisms may have the opportunity to evolve to much larger sizes. The island of Madagascar for example once housed Elephant birds - 400 kilograms and three terrifying meters tall. The development of such an imposing physique may be related to ecological release (the concept that in the absence of competition, a species may develop in a way that allows broader habitat and niche use). Competition between members of the same species becomes more important than competing with other species, and therefore being larger may mean access to better territory and food resources. Many examples of gigantism on islands exist. On the island of Minorca, around 3 million years ago, a massive rabbit relative Nuralagus rex outstripped its mainland relatives in size by twelve times. Along with a Minorcan giant tortoise, this lumbering rabbit dominated the landscape in the absence of predators or competitors.

Figure 2: Nurolagus rex as compared to modern rabbit

Naturally, however, a different rule must apply to large animals reaching islands. Well, actually, it’s the same rule, the so called ‘island rule’ which states larger species will evolve to become smaller on islands and smaller species will develop to become larger. Dwarfism is also prevalent on islands, and if we return to the example of Madagascar where the world’s largest bird became extinct, we find an entire genus of dwarf chameleons still wandering the leaf litter of the forest floors. Dwarfism may result from limited resource availability or dwarf organisms may simply be better adapted to obtain nutrients and efficiently utilise them than their larger counterparts.

Figure 3: Crotalus ruber 

A fascinating example of how resources and the pattern of colonisation can affect species size can be found on the island of Angel de la Guarda in the Gulf of California. Two species of rattlesnakes occur on the mainland near the island, the large species Crotalus ruber and its half-sized relative Crotalus mitchelli. On the island itself however, Crotalus ruber specimens are half the size of C. mitchelli, an exact reversal. The reason for this is thought to be that C. mitchelli dispersed to the island first where it became larger than its mainland counterpart, due to lack of competitors. When C. ruber arrived, the ‘large snake’ niche was already taken, thus limited resources caused a trend towards smaller specimens, until ultimately the snakes on the island filled the same niches as their mainland relatives, simply in reversed order.

The existence of species in isolated patches such as islands allows scientists a unique opportunity to study evolution and evolutionary processes on a manageable scale. It’s no co-incidence that both Darwin and Wallace came to their revolutionary ideas by studying island fauna. Islands exaggerate evolutionary processes in a way that allows for deeper understanding of the workings of processes like Natural Selection in a contained, natural laboratory.

Works Cited:

BROWN, J.H. & LOMOLINO, M.V. 1998. Biogeography. 2^nd Edition. Sinauer Associates Inc., Massachussets.

KAY, A.E. 1994. A Natural History of the Hawaiian Islands: Selected Readings II. University of Hawaii Press, Honolulu.

QUINTANA, J., KOHLER, M. & MOYA-SOLA, S. 2011. Journal of Vertebrate Paleontology 31(2):231-240. Available online at: http://www.bioone.org/doi/abs/10.1080/02724634.2011.550367 (Accessed 01/03/2013).

TYSON, P. 2008. Gigantism and Dwarfism on Islands. http://www.pbs.org/wgbh/nova/evolution/gigantism-and-dwarfism-islands.html (Accessed 01/03/2013).