A. The Seven Continents and the Seven Seas
Evolution is guided by the environment. To put our ancestors’ evolution in context, then, we should understand the changes taking place in their world.
By the end of Chapter 8, the continents had reached roughly their present-day configuration. There were a few key differences. Africa was completely isolated from other continents between 100 and 40 million years ago. 1 40 – 20 million years ago, Africa slowly coalesced with Eurasia. 2 A permanent juncture was formed at the Arabian Peninsula. Depending on land and sea levels, which fluctuated drastically, there was sometimes a land bridge at the Strait of Gibraltar. Even the Mediterranean Sea dried out at least twice. 3 Fluctuating sea levels also caused intermittent contact between North America and Europe and / or Siberia.
South America was isolated for tens of millions of years. It separated from Antarctica some time during the Oligocene Epoch. Since that time, a very cold circumpolar current has flowed from west to east around the Antarctic. There was once a parallel current between North and South America. South America’s northward drift closed that gap. Approximately ten million years ago, North and South America joined up at the Isthmus of Panama. The isthmus was formed gradually by the very forces of plate collision; it was not a lucky strike between two narrow necks of land! 4 That tiny isthmus had a big effect on ocean currents. The rich waters of the equatorial Pacific were no longer transported to the mid-Atlantic. On the Atlantic side, the warm tropical waters of the Caribbean Sea were redirected northward along the coastline of North America. This Gulf Stream brought warmer, moister air to northern Europe, making that region unusually warm for its latitude (consider that London, England is at the same latitude as Calgary, Canada). The Gulf Stream water and air is carried all the way to the North Pole, where the humidity precipitates and contributes to the northern ice cap.
Asia was the least-developed continent 30 million years ago. At that time, the southern continent was comprised of island arcs resembling today’s Indonesia and Philippines. The processes of plate tectonics, orogeny, erosion and sedimentation gradually filled it out. The Indian / Asian collision intensified around 25 million years ago, when the oceanic crust was completely subducted and the continents themselves butted up against each other. 5 Since then, India has been slipping beneath Asia, causing a massive uprising of land. The rugged Himalayan Mountains are the highest in the world. Just to their north is the Tibetan Plateau, also the world’s highest. The Himalayan-Tibetan region is now the site of large glaciers and the source of major rivers. The high wall of land intensifies South Asia’s annual monsoons. Summer clouds from the Indian Ocean dump all their moisture on the southern face of the Himalayas. The mountains leave a “rain shadow” to the north, making much of central Asia a vast desert.
Earth entered a significant cooldown period 15 million years ago. Carbon dioxide levels fell, 6 perhaps contributed by weathering of carbon-absorbing rock in the Himalayas. 7 The north and south poles both acquired their permanent ice caps during the Miocene, aided by the moist Gulf Stream to the north and the cold polar current to the south. By reflecting the sun’s heat, ice caps themselves contributed to further cooling. A cooler Earth is a more seasonal Earth, more prone to snow and ice each winter.
B. Modern Ecosystems
You probably remember studying the “food chain” in grade school: carnivores eat herbivores, which eat plants and fungi. Plant life is the foundation of the ecosystem. Plants, in turn, adapt to the climate and the geography of their environments. Thus, as the physical world – the continents, oceans, and atmosphere – made significant changes over the last ten million years, plant life also changed and animals followed suit.
The general trend over the last ten million years has been a cooler, drier Earth with a more diverse range of ecosystems. Plate tectonics produced major new mountain ranges on virtually every continent – the Rockies, the Andes, the Alps and Himalayas – which cast rain shadows to the inland side. The mountains themselves provided a greater range of high-altitude habitats. As the Earth cooled overall, jungles receded closer to the equator, thus becoming truly the “tropical” forests as we know them today. 1 This had great bearing on our ancestors, who occupied the dense forests of the Oligocene and Miocene.
In temperate latitudes, rain forests were replaced by woodlands and grasslands. In everyday speech, we use terms like “forest” and “woodlands” loosely and interchangeably. To an ecologist, a forest is what we would call a jungle, a densely populated range of evergreen trees. There is an overarching canopy of broad leaves over most of the forest, with shorter trees underneath. Forests are humid, shady, and rich in fruit and invertebrates. Woodlands are less dense and have only one layer of trees, which often lose leaves in the fall. A woodland floor is sunnier than a forest and more likely to support bushes and grass.
In plains with insufficient rainfall to support many trees, grasses dominate the landscape, punctuated by occasional groves of trees or bushes. The Miocene Epoch is characterized by the spread of grasslands and savannas, the boundaries between woodlands and grasslands. Grasslands helped accelerate global cooling by absorbing more greenhouse gases and emitting less water vapor than forests. 8
As the density of trees decreases, so does the density and diversity of animal life. Woods and grasslands offer a much less luscious menu than forests. The herbivores that thrive in grasslands have adapted to eating nutrient-poor grasses, seeds, and even bark. The Oligocene and Miocene saw a radiation of grazing hoofed animals such as camels, horses, and hippos. 9 Hoofed animals were highly successful worldwide and largely displaced the marsupials. The main exception was Australia, where hoofed animals never evolved and marsupials remained the dominant mammals. 10 Other large herbivores included elephants and sloths.
As for carnivores, the Oligocene Epoch saw the rise of dogs, cats, and bears. 11 An older form of carnivores, the now-extinct creodonts, also survived into the Miocene. The grasslands environment produced a literal arms “race” between herbivores and carnivores. With open space and little cover, running became a vital survival skill. Today’s antelope and cheetahs, which can run as fast as cars, are the result of tens of millions of years of chase on the African plains. Our primate ancestors were preyed upon by large cats and creodonts as well as snakes and birds of prey. 12
Sea life saw equal evolution revolutions. The primary new marine ecosystem of the Miocene was the kelp forest. 13 Whales and marine birds were at their most diverse during the Miocene. Newly evolved sea animals included seals, otters, and penguins.
- Ron Blakey and Tom Allinder, Tethys Ocean Through Time, https://vimeo.com/14507389 , 2011 (accessed 1/15/2017) ↩
- Rasoul Sorkhabi, The Zagros Uplift, GeoExPro, http://www.geoexpro.com/articles/2012/05/the-zagros-uplift (2012, accessed 1/15/2017). ↩
- Discussed (but not discovered) by Roveri et al, “The Messinian Salinity Crisis: Past and future of a great challenge for marine sciences”, Marine Geology 352 (2014) 25 – 58, http://www.geo.uu.nl/~forth/publications/Roveri_2014.pdf (accessed and saved 1/29/17) ↩
- Bacon et al, “Biological evidence supports an early and complex emergence of the Isthmus of Panama”, PNAS Vol. 112 no. 24 (6/16/2015), http://www.pnas.org/content/112/19/6110.full.pdf (accessed and saved 1/29/2017). ↩
- Van Hinsbergen et al, “Greater India Basin hypothesis and a two-stage Cenozoic collision between India and Asia”, PNAS vol. 109 no. 20 (5/15/2012), http://www.pnas.org/content/109/20/7659.full.pdf (accessed and saved 1/29/2017). ↩
- Herbert et al, “Late Miocene global cooling and the rise of modern ecosystems”, Nature Geoscience 9, 843-847 (2016), dio:10.1038/ngeo2813, abstract at http://www.nature.com/ngeo/journal/v9/n11/abs/ngeo2813.html (accessed 1/30/2017), summary at https://news.brown.edu/articles/2016/09/miocene (accessed and saved 1/30/2017). ↩
- Wilson, Drury, and Chapman, The Great Ice Age: Climate Change and Life, Routledge, 2000, summarized on p. 176, https://books.google.com/books?id=weKGAgAAQBAJ&pg=PA176 (accessed 5/07/2017). For an explanation of how weathering can absorb CO2, see John Mason, “Understanding the long-term carbon-cycle: weathering of rocks – a vitally important carbon-sink”, Skeptical Science, 7/02/2013, https://www.skepticalscience.com/weathering.html (accessed and saved 5/07/2017). ↩
- Gregory Retallack, “Cenozoic Expansion of Grasslands and Climatic Cooling”, The Journal of Geology, vol. 109 no. 4 (July 2001), pp. 407-426, for purchase at JSTOR, www.JSTOR.org/stable/10.1086/320791 (accessed and saved 1/18/17). ↩
- Thom Holmes, The Age of Mammals: The Oligocene and Miocene Epochs (Chelsea House Publishers: New York City, 2009), Ch. 4. ↩
- Thom Holmes, The Age of Mammals: The Oligocene and Miocene Epochs (Chelsea House Publishers: New York City, 2009), p. 26. ↩
- Thom Holmes, The Age of Mammals: The Oligocene and Miocene Epochs (Chelsea House Publishers: New York City, 2009), p. 147. ↩
- Donna Hart and Robert Sussman, Man the Hunted, Westview Press, 2005. ↩
- David Polly and Brian Speer, “The Miocene Epoch”, University of California Museum of Paleontology, 1994 – 1997, http://www.ucmp.berkeley.edu/tertiary/miocene.php (accessed and saved 2/19/2017). ↩
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