The Elements of Evolution (26) Geology & Plants

Geology & Plants

2 geological factors have been relevant to plant evolution: episodic orbital variations and tectonic changes, both gradual and dramatic.

Milankovitch Cycles

The Earth has wobbled on its axis over geological time. Serbian geophysicist Milutin Milanković proposed in 1920 what is now known as Milankovitch cycles: long-term climate changes as a product of fluctuations in Earth’s orbit (from more circular to more oval), the tilt of Earth’s axis of rotation, and the proximity of Earth to the Sun by time of year. These 3 factors run through cycles in annualized durations of about 95,000 (orbit), 42,000 (tilt), and 21,000 (distance).

When all 3 align, Earth’s climate swings to an extreme; either glacial (cold) or interglacial (warm). The global climate is between these extremes most of the time. Milankovitch cycles have paced global climate temperatures throughout Earth’s history.

The growth of ice affects more than the seas and land around it. During major glaciations, sea levels have dropped globally by up to 100 meters from water locked up in the ice caps. The water circulating in the atmosphere also declined.

Growing high-latitude glaciers created deserts in the tropics, with adverse impacts on both flora and fauna. The reverse is true for global warming taken to an extreme.

Global redistribution of vegetation correlates with Milankovitch cycles. Plants respond to Milankovitch cyclicity.

Unlike animal evolution, evidence indicates that Milankovitch cyclic extremes prune the number of plant species but do not stimulate speciation. A different driver is more closely associated with plant evolution.

Tectonic Pulses

On a planetary scale, the Earth’s lithospheric plates have shifted in identifiable pulses in geologic time. 5 episodes of especially enhanced tectonic activity have been recognized: 460–430 MYA (Ordovician–Silurian), 375–350 MYA (Devonian–Carboniferous), 300–260 MYA (Carboniferous–Permian), 170–160 MYA (Jurassic), and 120–80 MYA (Cretaceous). The 5 tectonic pulse periods are characterized by coalescence or cracking of continents. Regardless of the direction of tectonic movement, the pulses correlate with periods of increased species turnover.

These pulses had quickened rates of seafloor spreading, active volcanoes, and mantle degassing. The combined effects were greater greenhouse gases and eustatic sea level elevation which caused global warming and humidity. The enhanced humidity resulted from larger areas of shallow seas.

The mid-Ordovician–Silurian pulse coincides with the terrestrialization of non-vascular plants. Toward the end of this pulse came vascular plants.

During the mid-Devonian–early-Carboniferous pulse came the rapid evolution and expansion of plant families, culminating in the earliest seed plant groups. Continental drift was considerable during this time.

The supercontinent Pangea was created in the pulse of the late-Carboniferous–early-Permian. Seed plants (gymnosperms) rapidly evolved and radiated from 280–260 MYA.

The initial breakup of Pangea occurred in the mid-Jurassic. North America and Asia (Laurasia) separated from Africa/South America (Gondwana). The oceanic crust that would sport the Atlantic Ocean spread. No new major reproductive grades emerged then, but new gymnosperm orders evolved, including the family that evolved into angiosperms shortly thereafter.

The Cretaceous pulse saw rapid spreading episodes in the Pacific and Atlantic oceans. Pangea finished breaking apart, creating broadening ocean margins between land masses. There was a massive radiation of flowering plants during this time. In contrast, fewer major plant originations occurred during relative tectonic stability. Rapid changes in terrain and climate provoke plant diversity.

The past 65 million years appear at a glance to be an exception. There has been total species diversity and high turnover during the Cenozoic era, even as tectonics have been relatively stable; but the apparent bustle belied no major evolutionary advent. Angiosperms remain dominant. No new reproductive grade has emerged. The rise of grasses are arid-adapted angiosperms.

~32 MYA, plants evolved to a different technique to fix atmospheric CO2: a 4-carbon molecule (C4) in the 1st carbon fixation product, in contrast to 3-carbon molecule products in the C3 plants of yore, which evolved when humidity was higher. C4 plants are more efficient at carbon fixation under conditions of drought, hot temperatures, and limited nitrogen or CO2. (C4 photosynthesis may have first evolved ~300 million years ago, but the evidence of that is inconclusive.)

Grass species in other ways adapted for aridity are major employers of the C4 pathway. C4 plants compose only 3% of flowering plant species, yet account for ~25% of global terrestrial productivity. 46% of grasses are C4, accounting for 61% of the C4 species. Cacti and many carnivorous plants in the angiosperm order Caryophyllales are C4 employers. C4 independently evolved numerous times.

Another carbon fixation adaptation, a variant of C4, exists for dry conditions: CAM (crassulacean acid metabolism) photosynthesis. CAM plants, such as pineapples, keep their leaf pores (stomata) shut during the day to reduce moisture loss, but open at night to collect carbon dioxide. Most succulent plants employ CAM.

These are significant adaptations in plant efficiency under arid conditions, but in a broader evolutionary framework changes in carbon fixation are lower-order adjustments.

Continental drift changes the global volume of terrain and shifts land masses, both in separation and latitude distribution. These tectonic physical products have profoundly affected animal life, but their impact on plant life has been less telling. What has mattered most to plants in tectonic upheaval has been atmospheric alteration, especially changes in carbon dioxide.

Some plant species are initially ecstatic over enhanced CO2 in the air: increased efficiencies of photosynthesis, water, and nutrient use are apparent. This productivity increase is temporary, as other limiting ecological factors for growth and repair kick in. Over a longer term, certain species show decidedly negative responses to elevated atmospheric CO2. Adaptation takes at least a few generations.

Either up or down, rapidly changing atmospheric CO2 is a major abiotic stress to plants. CO2 starvation spurred angiosperm evolution in the Cretaceous. In contrast, earlier episodes of major plant evolution have been during surges in atmospheric CO2.

The level of atmospheric carbon dioxide is a global dynamic, and so its effect is at the planetary level. The rising level of CO2 in current times represents a serious strain which no plant will escape.