Why are the transitions from one geological time era to another marked by mass extinctions? The explanations all involve climate change and associated survivable conditions for some species. But somehow there is an additional sense that as time turns over, for certain species, 'time is up.' At present we are living in the Cenozoic Era, which means 'new life era.' This is the time when the continents moved to their current positions, after the extinction of the dinosaurs. The Cenozoic Era is sub-divided as well - we are in its second, Quaternary Period. And in that period, we are in the Holocene Epoch, which means 'entirely recent epoch.'
The movement of the continents away from each other has changed the oceans and the earth's climate. Wiki: "There are two types of global earth climates: Icehouse and Greenhouse. Icehouse is characterized by frequent continental glaciations and severe desert environments. We are now in the icehouse phase, moving towards Greenhouse. Greenhouse is characterized by warm climates. Both reflect the supercontinent cycle." Technically, we are in an Ice Age right now, called the Quaternary Glaciation. But the Holocene Epoch is considered a warm geological time within that Ice Age, or what's known as an interglacial period. Ice ages run on 100,000 year cycles (just like everything else it seems). In addition to plate tectonics, the earth's climate is affected by associated rises in atmospheric Carbon Dioxide (we're not helping that natural shift), wobbles in our planet's orbit, and shifts in the earth's magnetic poles.
So with all this going on, it's not surprising that the primary geological event of our Holocene Epoch is the Holocene Extinction.
Wiki describes the current era of mass extinction as follows: "Between 1500 and 2009, 875 extinctions have been documented by the International Union for Conservation of Nature and Natural Resources. However, since most extinctions go undocumented, scientists estimate that during the 20th century, between 20,000 and two million species actually became extinct, but the precise total cannot be determined more accurately within the limits of present knowledge. Up to 140,000 species per year (based on Species-area theory) may be the present rate of extinction based upon upper bound estimating."
The Quagga, an extinct sub-species of African Plains Zebra.
Some articles report that the earth is now undergoing the biggest extinction event in 65 million years, with over 50 species going extinct every day, which is 100 times normal rates for an extinction event. That's a conservative 1995 estimate (J. H. Lawton and R. M. May, Extinction Rates, Oxford University Press, Oxford, UK). Much of this is blamed on human activity. But the other chief cause is estimated to be climate change (also somewhat related to human activity). The meaning of the Holocene Extinction depends on which politically armed camp of researchers you consult, given their pitched battles between 'overkill' and 'overchill' arguments. It's a classic, very human problem of perception: is this all about us, or is this all about the cosmos? The latter perspective involves questions arising from our observations of Venus (in a supergreenhouse stage), a dead Mars, and the evolution of our sun. It might involve issues like meteor impacts, as with the Chicxulub event, which is hypothesized to have been the catalyst in the extinction of the dinosaurs.
A 2009 book summmarizes the Holocene Extinction and attempts to explain it (Samuel T. Turvey, ed. Holocene Extinctions, Oxford University Press, Oxford, UK). It's reviewed at the journal Integrative and Comparative Biology here. If extinctions are natural events, then we must ask if certain life forms are bound to certain time periods and the conditions that characterize them. That is a profound moral and philosophical question, in the sense that it goes beyond just looking at a particular era in time as a collection of environmental conditions that make certain life forms comfortable.
Do temporal epochs present different alignments of nature, physics and life that, taken together, constitute larger meanings, which can be said to literally characterize time periods? This might be the original derivation of our concepts of destiny, fate, and a vast family of spiritual and religious metaphors. These far-from-scientific symbols conceivably indicate our instinctive recognition that living creatures (including ourselves) belong to a particular time and place, and to remove them from it forcibly is somehow deeply wrong. It's an invisible boundary that we as a species find fantastically tempting. Hence we have cautionary sci-fi speculations like Jurassic Park which warn us that we can't, shouldn't and must not muck around with the natural course of time, even if we develop the technology (as with cloning) to bring back extinct species. This is really a recasting of Mary Shelley's Frankenstein; or, The Modern Prometheus, which forbids us from playing god and bringing back the dead.
Pangea Ultima. How the world might look 250 million years from now.
Our inability to conquer extinction relates to our psychological preoccupation with the immutability of death. The inexorable change of everything we understand to be unchanging goes to the heart of our deepest fears and uncertainties of our relation to the world and the degree of importance of our impact on it. Plate tectonics are a big example. How do you perceive reality when the ground is moving beneath your feet? Continental Drift Theory was first proposed by Alfred Wegener in 1912. If you're interested in the rash of earthquakes that seem to plague the planet every day now, you might want to check out where scientists see the continents ending up. They predict the creation of a new Supercontinent, called Pangea Ultima, (also called Pangaea Proxima, Neopangaea, and Pangaea II). The proposed alternative final configuration is the more Orwellian-sounding Amasia. Either projection would arise out of the Supercontinent Cycle, which runs on a 300 to 500 million year schedule.
And if this isn't enough, some researchers relate the expansion of the universe to pressures on the earth's plates and believe our planet itself is expanding. They theorize that the Earth's orbital eccentricity and continental drift on the planet, which so affect our climate and the evolutionary lifespans of species, are both related to movements of other bodies in space and their associated electro-magnetic and gravitational pulls. Maybe there is something to astrology after all!