The discovery of the Snowball Earth episodes suggests that temperature-induced events in planetary history may profoundly affect the course of bi-otic evolution. This argument can perhaps be extended not only to specific episodes of planetary temperature change but also to actual temperature values over time. Could cooling planetary surface temperature reaching some critical value have been the stimulus for other major breakthroughs in biological evolution?
As we saw in Chapter 2, the habitable zone is most commonly defined in terms of the presence of liquid water; this definition thus includes everything from life forms capable of living in boiling water to those capable of life in ice or snow. It may be that over much of its history, Earth was either too hot or too cold to allow the emergence of animals. Environments with temperatures near the freezing point or the boiling point of water are occupied largely by microbes; animals tolerate a much narrower temperature range. David Schwartzman and Steven Shore have pointed out that eukaryotic organisms with mitochondria (the organelles that convert fuel into energy) have an upper temperature limit for viable growth of 60°C. This limit is apparently determined by the chemical structure of the mitochondrial wall. Because eukaryotes evolved from prokaryotes, the habitable zone of a planet is narrowed from the region that allows the presence of water (0—100°C) to the narrower range of 0-60°C. Schwartzman and Shore note, "We assume that the emergence of relatively simple life forms is almost inevitable on Earthlike planets. Such organisms are remarkably robust. Complex life, however, requires a more restrictive set of physical conditions—in particular, lower temperatures."
Schwartzman and Shore provided the following list of the critical upper temperatures for various organisms on Earth.
Approximate Upper Time of First Appearance
Group Temperature Limit (°C) on Earth (billions of years ago)
Multicellular plants 45-50 0.5
Animals 50 1-1.5
Eukaryotic microbes 60 2.1-2.8 Prokaryotic microbes
Cyanobacteria 70-73 3.5
Methanogens >100 3.8
Extreme thermophiles >100 3.8
Schwartzman and others have proposed that Earth's surface temperatures have been the critical constraint on microbial evolution, determining the timing of major innovations. They believe that when Earth's surface cooled below 70°C more than 3.5 billion years ago, cyanobacteria were able to evolve. These microbes colonized the surface of the land and, in so doing, increased weathering rates and soil formation. The new soil in turn acted as a sink for removing carbon dioxide from the atmosphere, thereby causing further cooling of the planet. Each innovation among microbes resulted in biotic enhancement of weathering. This process has been dubbed "biotically mediated surface cooling." With the evolution of higher plants with their often elaborate root systems, this process increased greatly in efficiency. It is at the heart of the "Gaia Hypothesis," wherein Earth is conceived of as a self-regulating "superorganism," a perspective shared by many scientists, including Lynn Margulis, Tyler Volk, and the originator of the term, James Lovelock. We remain agnostic on this particular interpretation but see much merit in the view that the emergence of animals may have been strongly influenced by surface temperatures—and that life itself on this planet has had an enormous impact on planetary temperatures.
Could there be any way in which (or any planet whereon) animals could evolve faster than they did on Earth? The physical events affecting Earth immediately before the emergence of large, skeletonized animals were among the most complicated in all of Earth history. Was this just coincidence, or did it make the acceleration of animal evolution possible? These questions, and the curious and dramatic fashion in which the major body plans of animals suddenly began to commonly appear in the fossil record on our planet about 540 million years ago, are the topics of the next chapter.
The Enigma of the Cambrian Explosion
Evolution on a large scale unfolds, like much of human history, as a succession of dynasties.
—E.O. Wilson, The Diversity of Life
Our planet was without animal life for the first 3.5 billion years of its existence and was without animals large enough to leave a visible fossil record for nearly 4 billion years. But when, 550 million years ago, sizable and diverse animal life finally burst into the oceans, it did so with a figurative bang—in a relatively sudden event known as the Cambrian Explosion. Over a relatively short interval of time, all of the animal phyla (the categories of animal life characterized by unique body plans, such as arthropods, mollusks, and chordates) either evolved or first appear in the fossil record. Undoubted fossils of metazoan animals have never been found in 600-million-year-old sedimentary strata, no matter where on Earth we go.
Yet the fossils of such animals are both diverse and abundant in 500-million-year-old rocks, and they include representatives of most of the animal phyla still found on Earth. It appears that in a time interval lasting at most 100 million years (and in fact, as we will see, in an interval considerably shorter than that), our planet went from a place without animals that could be seen with the unaided eye to a planet teeming with invertebrate marine life rivaling in size almost any invertebrate species on Earth today. This follow-up to the initial animal diversification of more than 700 million years ago (described in the last chapter) is the Cambrian Explosion.
The rate of evolutionary innovation and new species formation during the Cambrian Explosion has never been equaled. The prior animal diversification must have involved very few species, each growing to a very small size; the Cambrian Explosion, on the other hand, produced huge numbers of new species, many with completely novel body plans. The Cambrian Explosion presents a great challenge to astrobiology, as we will show in this chapter. Questions abound. For instance, can there be animal life on a habitable planet without this type of event? Is the Cambrian Explosion an effect or a cause? That is, could it be that the remarkable animal diversity on Earth today is a by-product of this sudden diversification and would not have come about if the Cambrian event had been a mild bang rather than an explosion? Was it inevitable once the late Precambrian first event had occurred, or was another set of stimuli required? What animals were involved? What were the event's biological origins? What caused it to occur? (Was there some sort of biological or environmental trigger?) And, most relevant to astrobiology, was the Cambrian Explosion inevitable once a certain level of biological organization had evolved? In other words, is there any way that the Cambrian Explosion might not have occurred?
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