Hi, I’m Bob Stern, I’m Taras Gerya, and we’re very pleased to participate in
this session “Unconventional Ideas and Outrageous Hypotheses: In Honor of Warren B. Hamilton”.
In this talk, entitled “The Plate Tectonic Pump: How the Transition from Single Lid to
Plate Tectonics Stimulates Biological Evolution” we explore how changes in Earth’s tectonic
regime are likely to have affected biological evolution. It is appropriate to do this in
a session honoring Warren Hamilton’s career. Warren was one of my heroes as an undergrad
at UC Davis in the early 1970s, where I read and reread the paper “Mesozoic California
and the Underflow of Pacific Mantle”. In this paper Warren explores how the fundamental
plate tectonic process that a year later would be called “Subduction” shaped California
in the Mesozoic. Warren continued contribute to our science well into his 90s, giving all
of us hope that we can live that long and stay so sharp. Our interests aligned again
in the last 15 years of his life, as we both wrote about when plate tectonics began and
what was Earth’s tectonic regime before that important transition. I was very excited
when he wrote in a 2013 email “I have no comprehension of how plate tectonics began,
except that I vaguely perceive it as a fuzzy transition from about 1 billion years ago to about 600 million years ago at the end
of which plate tectonics burst forth in its full splendor.” In the rest of this talk we will explore some
implications of the “fuzzy transition” for the evolution of life.
We know that Earth’s life and tectonics influence each other. Life on Earth began
sometime before 3.8 billion years ago, but we are more uncertain about when plate tectonics began.
Estimates range from the Hadean to the Neoproterozoic. Because 70% of the estimates for when plate
tectonics began are younger than 3.8 billion years ago and because we agree with Warren
Hamilton that plate tectonics began in Neoproterozoic time, we will assume that the “fuzzy transition”
occurred after life began. We focus not on what that “fuzzy transition” was but on how
it accelerated life evolution. What was Earth’s tectonic style before plate
tectonics started? It was almost certainly some kind of “Stagnant lid tectonics”. This is an
unfortunate term because the term “stagnation” implies that nothing is happening – think
of a stagnant career or a stagnant economy or a stagnant relationship. But that’s not
true for tectonically active silicate bodies with stagnant lid tectonics. Sure, some silicate
bodies with stagnant lid tectonics can be tectonically dead, like the Moon or Mercury,
but others are very active, like Venus. We think it is better to use a more generic term
“single lid tectonics” that includes both rigid, non-deformable lid and squishy, tectonomagmatically
active lid. There are five tectonically active silicate bodies
in our Solar System, and four of them have tectonomagmatic regimes that can usefully
be called “Single Lid”. But their styles all differ: Io has the most active volcanoes
and rapid magmatic resurfacing, Venus is vigorously erupting and deforming, with a squishy single
lid, and Mars has a sluggish single lid style. Earth before 3 billion years ago almost certainly
had a squishy lid tectonic style similar to Venus. After that, Earth may have experienced
a long transition from squishy lid to modern-style plate tectonics and may have had multiple different
styles of active single lid behavior. Kent Condie will talk tomorrow about the sluggish
Mesoproterozoic single lid magmatotectonic style.
What difference does tectonic style make for the evolution of life? First we need to think
about how evolution happens. We can see from the history of life on Earth that this occurred
in 3 stages, each taking different amounts of time and happening in different environments.
First, 80% of the 3.8 billion years it took for complex life to evolve was consumed by
starting life and evolving increasing complex single-cell and colonial organisms like stromatolites.
Stage 1 also led to an oxygen-rich environment, essential for evolution of larger, more complex
animals. Stage 1 evolution happened at different depths in the ocean, first around deep sea
hydrothermal vents and then in the shallow photic zone once photosynthesis evolved. Next,
“stage 2”, involving about 10% of “evolution time”, witnessed the development of multicellular
plants and animals, including fish with backbones and limbs. Because photosynthetic plants were
the base of the food chain, this stage mostly happened in the shallow water of the continental
shelves. Finally, “stage 3”, involving 10% of evolution time witnessed colonization
of land by plants and air-breathing animals. As we understand it, evolution of complex
organisms requires oceans, continental shelves, and continents. With this perspective in mind,
we now need to discuss how the tectonic style of active silicate bodies will affect these
stages. Once life begins on an active planetoid with
continents and oceans, evolution on a single lid body will proceed more slowly than on
a similar body with plate tectonics. This point was addressed 50 years ago in a seminal
paper by Jim Valentine and Eldridge Moores. They argued that the diversity of invertebrate
marine life – which flourishes on continental shelves - increased when the continents and
their shelves were fragmented, for example in Ordovician through Carboniferous or in
Cenozoic time. This was because continental breakup separated continental shelves, providing
the isolation needed to evolve new species. In contrast, when the continental fragments
recombined in Permo-Triassic time into a supercontinent surrounded by a single continuous continental
shelf, there was less diversity and evolution. A recent paper by Zaffos et al. confirmed
the findings of Valentine and Moores. From these results, we can extrapolate that
evolution on single lid bodies with otherwise identical distributions of oceans and continents
will proceed slower than for bodies with plate tectonics. The single lid body cannot easily
isolate environments needed for evolving new species in the same way that a body with plate
tectonics can. A simple thought experiment for two such bodies shows this. The panels
on the right are for the body with plate tectonics and the ones on the left are for the single
lid body. Both bodies start off with one type of plant, one kind of herbivore, and one type
of carnivore. Both bodies are similarly affected by meteorite impacts and mantle plumes, both
of which can cause extinctions and thus drive evolution, but the plate tectonic planet has
a way of isolating continents and continental shelves whereas the stagnant lid body does
not. Over the approximately 500 million years of a supercontinent cycle, rift and drift
associated with supercontinent breakup allows the isolation needed for new species to evolve,
and recombination causes organisms occupying the same ecological niche to compete. The
end result is that evolution will happen faster on a body with plate tectonics than it will
on a stagnant lid body. The role of plate tectonic isolation and recombination
for accelerating evolution may not be as important for stage 1 evolution as it is for stages
2 and 3. Single-celled organisms are more difficult to isolate by plate tectonic processes
because they are easily moved by water currents. In contrast, multi-celled organisms are harder
to move by currents and can be isolated by continental drift. It is still controversial
when plate tectonics began on Earth, but all parties to the controversy agree that plate
tectonics was happening during stages 2 and 3 on Earth.
There are other reasons why evolution is enhanced by plate tectonics and inhibited by single
lid tectonics. One reason is that Earth’s climate is buffered over long timescales by
a negative feedback between atmospheric carbon dioxide and surface temperature. Mantle degassing
as a result of igneous activity and metamorphism constantly adds CO2 to the atmosphere and
the greenhouse effect this produces over time would result in a climate inhospitable to
life unless there is a way to remove this greenhouse gas. Weathering – which removes
atmospheric CO2 – increases when the climate is warm, leading to a weaker greenhouse effect
and cooling surface temperatures. When surface temperatures are cooler, weathering is reduced,
allowing mantle-derived CO2 to build up in the atmosphere, leading to a stronger greenhouse
and warmer climate. Key for this negative feedback loop is a constant supply of fresh
rock for weathering. Plate tectonics supplies this in abundance via basaltic lavas associated
with hot spots and rifting, lavas and ash from arc volcanoes, and crust exposed by continental
collision and rift-related uplift. Single lid planets are less capable of generating
weatherable rocks and so are more likely to have climates that are inhospitably warm for
life. Nutrient supply is also important for life
and evolution. Life requires a long list of elements such as Carbon, Oxygen, Nitrogen, Phosphorous, Iron, and many
other trace elements that are contained in rocks. These elements cycle between life on Earth’s
surface and the mantle as a result of plate tectonics, especially via outgassing and volcanism and
by weathering of rocks. These interactions between tectonics and life are the focus of
a new interdisciplinary research field called biogeodynamics. We are beginning to understand
biogeodyamic interactions associated with plate tectonics on Earth but have not even
begun to explore how biogeodynamics would occure on active single lid planets. It does seem likely
that the supply of nutrients to the biosphere is stronger with plate tectonics than without.
Let’s put all these ingredients together to think about what is likely to happen on
a planet with life when its tectonic regime changes from single lid to plate tectonics.
For the reasons articulated previously, evolution is likely to be slow during single lid episodes.
Because plate tectonics easily causes isolation and recombination, moderates climate, and
enhances nutrient supply, evolution is likely to accelerate with a transition to plate tectonics.
Let me repeat this key conclusion: Evolution rates on a planet with life will be slow when
it has a single lid tectonic style and will accelerate when it has a plate tectonic style.
Of course, there is much that we don’t know about how planetary tectonic style is likely
to affect rates of biological evolution, for example what are the effects of changing tectonic
style for Stage 1 evolution compared to Stage 2 or 3. For now it may be enough to consider
that this is a very promising line of future research where biologists and geoscientists
can work together. Cross-disciplinary work is needed to assemble and analyze critical
observational data as well as to develop new hybrid numerical bio-geodynamical modeling
tools coupling geodynamic processes to evolution of atmosphere, oceans, climate and life.
One last point worth considering is what these conclusions mean for the search for extraterrestrial
life. Technological species – defined as those capable of radio communications – are
more likely to evolve on a silicate planet with plate tectonics than on a single lid
planet. When we are able to infer the presence of life and the tectonic regime of habitable
exoplanets, it may be better to colonize the single lid planets with life than the plate
tectonic planets. We are more likely to repeat the bad behaviors associated with colonizing
the New World and Africa during the past few hundred years here on Earth on plate tectonic
exoplanets than on single lid exoplanets. Thank you for listening.