The profusion of different kinds of organisms has been increasing, on average, for hundreds of millions of years. That’s a long time. Things did appear to plateau for awhile (for reasons that remain unclear, from around 450 million to around 250 million years ago). But since then it’s been back to normal, with the number of different species following a generally increasing trajectory punctuated by occasional downward plunges indicating mass extinctions. These dramatic extinctions have a curious tendency to occur about 62 million years apart.
Although the jury is still out on why, one interesting theory is that changes in atmospheric or oceanic oxygen levels stress the biosphere, leading to mass extinctions. Genetic markers suggesting this indeed have a periodicity component of close to 62 million years. Reductions in ocean currents can cause the oceans to stagnate, in turn leading to loss of oxygen in the water, so this is one plausible theory about these mass extinctions. Based on the 62 million year cycle time, it appears we are due for a mass extinction right about about now, give or take a few hundred thousand years. And in fact, one is happening now. This one is due to human-caused activities such as cutting down rainforests (and more generally, habitat destruction), unsustainable poaching, and climate change. Hopefully, ocean currents and other oxygen level regulating processes remain stable through this or things could get even worse. However, human caused global warming is thought by some climatologists to present a significant plausible risk of just such an ocean current stagnation.
The underlying tendency posed by the 62 million year cycle, the current new human-caused climate warming trend, and the on-going wave of extinctions caused by habitat destruction present a bad luck trifecta that gives pause. However, human influence is decidedly short term from the perspective of geological time scales of 100 million years or more, making the current situation only a temporary disturbance in an apparent long term trend of increasing species diversity. Long, long after we humans are either extinct ourselves, or have changed considerably more that the difference between what we are today and gorillas, lemurs, or even squirrels, the sun will still rise daily at dawn and set at dusk. And hopefully life on Earth will continue on for billions of years as it has for billions of years already. This brings us back to the long term trend of increasing species diversity, despite the occasional negative influences of mass extinctions.
Check out part 2, to be posted soon, on “How Species Come to Be”…same bat time, same bat channel
“The profusion of different kinds of organisms has been increasing, on average, for hundreds of millions of years.” Http://www.mun.ca/biology/scarr/Extinction_rates.html, 5÷29÷10; http://en.wikipedia.org/wiki/File:Phanerozoic_Biodiversity.svg, 5÷29÷10; A. V. Markov and A. V. Korotayev, Phanerozoic marine biodiversity follows a hyperbolic trend, Paleoworld (Dec. 2007), vol. 16, issue 4, pp. 311–318.
“These dramatic extinctions have a curious tendency to occur about 62 million years apart.” R. A. Rohde and R. A. Muller, Cycles in fossil diversity, Nature, Mar. 10, 2005, vol. 434, pp. 208–210.
Figure: “Number of marine invertebrate families through prehistory, based on the fossil record (after Sepkowski 1984; Miller and Foote 1996.” (1) J. J. Sepkoski, Jr., A kinetic model of Phanerozoic taxonomic diversity, III. Post Paleozoic families and mass extinctions, Paleobiology (1984), vol. 10, pp. 246–267. (2) A. I. Miller and M. Foote, Calibrating the Ordovician radiation of marine life — implications for Phanerozoic diversity trends, Paleobiology (1996), vol. 22, pp. 304–309.
“Genetic markers suggesting this indeed have a periodicity component of close to 62 million years.” G. Ding, J. Kang, Q. Liu, T Shi, G. Pei, and Y. Li, Insights into the coupling of duplications events and macroevolution from an age profile of animal transmembrane gene families, PLoS Computational Biology, 2006, vol. 2, no. 8, e102.