Speciation Pumps
One of the most famous sentences in Darwin is the very last sentence of On the Origin of Species, which ends:
“...whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
Darwin here presents gravity as a fixed and unchanging background to the changing events on the surface of Earth, and as far as gravity alone is concerned, this is true, but many of the processes that characterize the Earth, changes such as the particular orientation of the Earth in relation to the sun, which is a function of gravity, which might also be thought of as a fixed and unchanging background to terrestrial evolution, in fact play a constitutive role in the beautiful and wonderful forms that have been, and are being, evolved.
Periodic changes over geological time according to which our planet goes on cycling sometimes act as what I will call speciation pumps, i.e., they act to force changes in natural history to which existing lifeforms must adapt, and, in adapting, organisms sometimes (although not always) become more complex, because, as Stephen Jay Gould has observed, complexity is the only degree of freedom away from the unrelieved simplicity of the baseline of life.
Recently in The Apotheosis of Emergent Complexity I compared the big picture views of history of David Cristian, who emphasizes emergent complexity, and Stephen Jay Gould, who emphasized the continuity of simplicity. Gould argued that there is no trend toward complexity in life, and while I concede every detail of Gould’s argument, part of it comes down to how one could identify a “trend” in natural history. The unspoken background of Gould’s argument is the rejection of teleology, progressivism, or even some kind of vitalism that assumes that life on Earth is pressing forward toward ever greater complexity.
It is possible, however, to disavow all forms of teleology and progressivism and still to recognize that certain Earth processes force the development of greater complexity, and as long as we do not imagine this complexity to be a goal toward which life is working, or imposed by an outside force, we can recognize this development of greater complexity as a trend in the history of life -- a trend that grows as long as continuity is maintained and previous complexity can be the basis for the later emergence of greater degrees of complexity. This process of development can be brought to an end at any time by a catastrophic denudation event of lesser or greater severity, and its outcome is not predetermined in advance, so there is no inevitability involved, but as long as a biosphere is allowed to cycle on, subject to speciation pumps, greater complexity will emerge.
The two primary speciation pumps that I see at work in terrestrial life are supercontinent cycles and Milankovitch cycles. Supercontinent cycles are another way to talk about geomorphology or plate tectonics. The supercontinent cycle, according to which continental plates are slowly assembled into a single, vast landmass and subsequently broken apart, is the largest and longest cycle of the many cycles of Earth processes described by geomorphology and plate tectonics (examples of less comprehensive cycles would include the rock cycle or the hydrological cycle). These forces act over periods of hundreds of millions of years to assemble and disassemble supercontinents, but some of the movements of continental plates can have significant impact over shorter periods of time. For example, only two million years ago (with human ancestors already well on the road to rapid encephalization), the Great American Interchange occurred when North America and South America, previously separated, were joined by the rise of the isthmus of Panama (Central America).
The sequence of supercontinents resuting from the supercontinent cycle include Vaalbara (~3.6 Ga. ago), Ur (~3 Ga. ago), Kenorland (~2.7 Ga. ago), Columbia, also known as Nuna (~1.8–1.5 Ga. ago), Rodinia (~1.1 Ga–~750 Ma. ago), Pannotia, also known as the Vendian supercontinent (~600–~545 Ma. ago), and Pangaea (~300–~210 Ma. ago), though the further back we regress in time the more difficult it is to reconstruct past supercontinents. As the tectonic plates rearrange themselves over geological time the concept of longitude ceases to have meaning, although a paper from a few years ago, Supercontinent cycles and the calculation of absolute palaeolongitude in deep time, argues that it may be possible to construct, “absolute palaeolongitude over billion-year timescales.” Work such as this will allow for a more accurate reconstruction of past supercontinents.
The Milankovitch cycles involve orbital eccentricity, axial tilt, axial precession, and apsidal precession. Each of these cycles has its own periodicity, and working out over the long term their synchronization and and desynchronization create further cycles. The upshot of Milankovitch cycles is a change in the seasons and in the amount of sunlight that the Earth can absorb. This, in turn, changes the temperature on Earth, and temperature changes mean climatological changes.
One example of how speciation pumps work on Earth can be furnished by cycles in sea levels. When global temperatures increase, ice melts and sea levels rise; when global temperatures decrease, water freezes as ice, and sea levels fall. If temperatures go low enough, accumulated ice weighs down on the continents and pushes them lower into the crust, so that there are interactions between climatological changes primarily driven by (or, if you like, “pumped” by) Milankovitch cycles and geomorphological forces. On the edges of continents, continental shelves will be exposed when sea levels are low, and immersed when sea levels are high. Coastal mountain ranges can be changed into an archipelago of islands when sea levels rise. The plants and animals that live along the coasts are forced to adapt (or go extinct) repeatedly as sea levels rise, forcing surviving species onto island refugia, and then sea levels fall, reconnecting former islands and allowing the now allopatrically speciated plants and animals to mingle in the lowlands revealed by receding waters. Repeat this process every hundred thousand years or so (as has happened through the current Quaternary glaciation), and you have surviving populations that have repeatedly adapted. Over millions of years this drives considerable biodiversity.
These speciation pumps at work on Earth -- supercontinent cycles and Milankovitch cycles -- should be understood as two cyclical selection pressures, but not as necessarily the only large scale cyclical selection pressures that might act as speciation pumps. The point is that speciation pumps might be at work in other biospheres on other planets, but these speciation pumps might not be (probably would not be) identical to the speciation pumps that that driven emergent complexity in the terrestrial biosphere.
For example, it is likely that any planet anywhere in the universe that has a biosphere has something like Malenkovitch cycles, because no planet is likely to have a perfectly circular orbit, no precession, and unvarying axial tilt. But Milankovitch cycles might vary quite significantly from what we experience on Earth. A planetary orbit might be more or less eccentric and axial tilt might vary more or less.
We can conduct a thought experiment in a counterfactual to the history of our own planet, namely, a planet with a biosphere that is perfectly consistent and perfectly quiescent. By this I mean a planet that always has the same orbital periodicity, distance to its star, and axial tilt, and which experiences no geomorphological upheavals, with an unchanging circadian rhythm and an unchanging climate at each respective latitude and altitude.
Such a planet and its biosphere would be exposed to selection pressures, but these selection pressures would remain consistent over time. In this way, the selection pressures would be more like parameters within which life might flourish or fail, but which would not change the direction of the development of once life had gotten started within these parameters. We could call this a non-selective natural history, although strictly speaking this would be inaccurate, as selection would continue (especially stabilizing selection). Such a counterfactual biosphere not driven by cyclical selection pressures might remain at a minimum level of biological complexity in the absence of speciation pumps.
The distinctive feature of a speciation pump is that it is a recurrent (and perhaps also regularly periodic) selection pressure that acts over a period of time over which biological organisms can adapt. A periodicity that is too long would not “pump” the biosphere, forcing populations to repeatedly adapt, while a periodicity of too short a period of time would not allow natural selection to function over the periodicity of the life cycles of terrestrial organisms. In other words, a speciation pump needs to be “just right” and thus constitutes another “Goldilocks” condition for the emergence of complex life.
Recently in my revised post Existential Lessons of the Cold War I quoted a passage from the Soviet physicist Igor Kurchatov, warning of the dangers to terrestrial life as a result of a nuclear war and the residual radiation from a nuclear war:
“There is no hope that organisms, and the human organism in particular, will adjust themselves to higher levels of radioactivity on earth. This adjustment can take place only through a prolonged process of evolution.”
If a planet experienced periodic changes in radiation levels that occurred over periods of time sufficient for adaptation, this too could act as a speciation pump, but a single catastrophic event resulting from a nuclear war, like the K-Pg extinction event, whether triggered by the Chicxulub asteroid impact or supervolcanoes (or both), is rather a denudation event. While a mass extinction can generate speciation and biodiversity in subsequent generations when adaptive radiation fills the void left by denudation and mass extinctions, these events function according to a mechanism distinct from a speciation pump, although over the very long term (the same multi-billion year scale of time as supercontinent cycles) repeated mass extinctions could be considered the largest scale expression of a speciation pump.
As a side note to Kuchatov’s warning, one would suspect that a radiation-tolerant adaptation would be much like a mangrove swamp, which consists of salt-tolerant vegetation. Adaptations of this kind often involve considerable compromises for the organism to live under conditions otherwise hostile to life. In effect, hardened life forms that can survive in saline environments, or presumably also in high radiation environments, are forced toward becoming extremophiles in order to survive. If the environment becomes even more hostile, even the extremophile may be forced into extinction, but where the environment becomes less hostile to life we generally do not see an adaptive radiation of the extremophile, because such a high cost is paid for remaining alive in a difficult environment. When a mangrove swamp becomes less saline, other plants and animals less tolerant to salt move into the environment in a process of ecological succession.
We should think of the periodic selection pressures I have called speciation pumps (i.e., the two I have named above) as but two instances of a class of periodic selection pressures. Other worlds with biospheres may have other periodic selection pressures that act as speciation pumps in their respective biospheres. For example, a star that varies in brightness according to a fixed period over biological or geological time might force speciation events on a regularly recurring basis on the planets or planets that orbit such a star. Or consider the subsurface ocean worlds of the Jovian moons: interactions with a gas giant planet or with other moons might function as a speciation pump in such environments. In this way, we would not need to see a nearly precise Earth twin, complete with plate tectonics, in order to find the emergence of complex life elsewhere. Any speciation pump will do, although one suspects some speciation pumps are more effective than others. It is to be hoped that it has not escaped the reader that the existence of a class of speciation pumps, some present on Earth and some not, bears upon the rare Earth hypothesis.