Does SETI Make Sense? Part III: Evolution

Harry SETI header

Every planet that develops life based on chemistry similar to ours will begin with single cells. The entire water ecosystem will consist of these cells in some variety. That variety necessarily came about due to errors in copying the cells from one generation to the next. They would have a rather mundane life of drifting about randomly until encountering some useful molecule and absorbing that molecule. When enough of these molecules had been absorbed, possibly taking years, the single cell will divide into two.

In this slow, inexorable process, the seas will become full of these cells. Some will drift to inhospitable places where they’ll be killed and spill their contents back into the sea for other cells to absorb. Direct conflict is unlikely because the apparatus for killing and absorbing other cells is too complex to develop readily.

Altogether, there’s something like a chance in a billion that a given star will have a planet that can develop and sustain life. The chances are probably much worse.

Early on, after about a billion years, some developed the ability to use sunlight to make molecules from CO2 and water, from chlorophyll, probably an early form that has evolved into its many varieties today. Some scientists suggest that the earliest versions of chlorophyll did not produce oxygen as a byproduct. By about 2.3 billion years ago, some definitely had and started putting oxygen into the water. For life at that time, oxygen was a serious poison, worse than cyanide is to us. It was a matter of adapt or die — or hide somewhere where oxygen did not exist.

This was probably the first great extinction on Earth, and it was caused by oxygen pollution. Evolution favored those who had a way to neutralize this nasty chemical. Slowly but surely, the removal of so many anaerobic species left ecological niches open, and aerobic cells began to fill them. They used a new way to create energy though oxidation, a much more efficient way than their predecessors. Unfortunately (or fortunately from our viewpoint), the oxygen had some other side effects.

As oxygen was pumped into the water, it was quickly absorbed by chemical reactions in the then reducing environment, but this absorption could last only so long. Finally, the oxygen escaped into the atmosphere, which contained plenty of methane and CO2. Methane is a powerful greenhouse gas, more so than CO2. Oxygen reacted with the methane readily, producing water and CO2. The Earth cooled. Scientists say that this sequence triggered the great Huronian glaciation, the first snowball Earth.

The evidence is powerful and pretty much accepted today that the Earth was essentially covered with ice from pole to pole at least once and maybe more times. Some suggest that the equatorial belt was free of ice at least part of the year. It’s hard to distinguish full coverage and nearly full coverage.

If you accept this reasoning, then you expect that no more than one in ten trillion stars have life on them and that it could be even 100 times fewer. In terms of galaxies harboring life, this result means that one galaxy in 100 has intelligent life.

Whichever was the case, the changed environment challenged life on Earth as it had not been challenged before. No light penetrated to the water under the thick ice layers. Life as usual was no longer possible. Cooperation became necessary to survival in many of the habitats under the ice. A second effect of oxygen in the atmosphere was the creation of an ozone layer that shielded the land from damaging ultraviolet radiation. Finally, oxygen in the environment allowed the development of more complex life, especially the eukaryotes, that used more efficient aerobic metabolism.

Eukaryotes, which probably developed around two billion years ago have an internal organization, unlike earlier prokaryotes that consisted of just a bunch of molecules in a bag. Both aerobic metabolism and eukaryotic cells were necessary for the next great step.

While controversy continues regarding exactly how it happened, there’s no question that multicellular organisms appeared after the last snowball Earth episode and that, about 100 million years later, the great Cambrian explosion (ca. 542 million years ago) took place with the sudden appearance of a great variety of fossils that stumped paleontologists for a very long time.

Different classes of animals dominated the Earth at different times. Beginning only around 20 million years after the Cambrian explosion, trilobites began to populate the seas. This class of animals existed on Earth for over 270 million years before disappearing in the biggest mass extinction on record at the end of the Permian, about 250 million years ago. At that time, some 96% of all species were destroyed. The late Devonian extinction at around 364 million years ago had already reduced their variety considerably, however. Until then, Earth was a trilobite world.

The Permian (or Permian-Triassic) extinction allowed dinosaurs to take over the Earth. Despite minor extinction events afterward, or possibly because of them, dinosaurs dominated every large-animal niche on land and many in the sea. Their end famously came 65 million years ago when a large solar body impacted the Earth, and all non-avian dinosaur species were obliterated after a run of nearly 200 million years.

Fortunately for us, the mammals were ready to take over. At the time of the dinosaur extinction, mammals were quite small and either burrowing or tree-dwelling creatures. Mammals rapidly radiated into a wide variety of species, including our predecessors, the primates. Note that the dinosaurs had nearly 200 million years to develop intelligence and didn’t even get started. Instead, they engaged, evolutionarily speaking, in a toughness arms race as they grew larger, added claws, added armor, and found ways to run faster. Bipedal dinosaurs showed no tendency to abandon their claws and develop manipulative digits. Their brains remained the size of walnuts. I doubt that another 200 million years would have changed that. Were it not for that cataclysmic collision, the Earth would almost certainly still be a world of dumb dinosaurs and would not be beaming radio signals into space.

What is the likelihood of each of the steps toward unicellular life becoming us?

The initiation of life seems to require that a planet be large enough and within a range of distance from its sun where liquid water can flow on its surface. Because rocky planets have been discovered circling stars that were born early in the life of the universe, under one billion years, nearly any star is a candidate. Larger stars, though, do not last long enough to support advanced life on their planets. If a star is too small, it will not have enough heat to warm its planets above freezing. These criteria must eliminate about half of all stars.

The place of a star in its galaxy also affects its ability to support life over billions of years. Stars near the center of a galaxy or in the thick part of the arms of a spiral galaxy are too close to occasional novas and supernovas that sterilize them with powerful radiation. Again, this criterion will eliminate at least half of all stars.

The situation with the Earth that its atmosphere was ripped away by a major collision late in its development was crucial to long-term life. Its methane and CO2 were removed and a runaway greenhouse effect prevented. Early in the life of a solar system, such collisions must be frequent, but later on they will dwindle to nearly zero as planets coalesce. This collision at this time in Earth’s history certainly is a million-to-one shot and possibly even less likely.

There’s also the likelihood that an appropriate planet will circle in an orbit that provides just the right warmth and the planet will be large enough to sustain a magnetic field for billions of years without its core solidifying and killing that important shield to solar wind. Mars is too small. This probability may be one in one hundred to take the optimistic side of the odds.

Altogether, there’s something like a chance in a billion that a given star will have a planet that can develop and sustain life. The chances are probably much worse.

Next, consider developing that life from unicellular into multicellular. How likely was the snowball Earth to happen? The evidence suggests that it took place as a direct result of the oxygenation of the Earth’s atmosphere by cyanobacteria. It makes sense that life would evolve to take advantage of sunlight, a copious source of energy, but does it follow that in so doing it would convert CO2 into oxygen? To be safe, assume that the answer is yes and that any such planet will experience a snowball freezing at the correct moment, which will stimulate development of multicellular organisms.

The multicellular life will evolve in the seas and move onto the land. This is quite certain, although the land will take a very long time after the seas are well populated. Will the land welcome life? Without our large moon, as previously indicated, winds on Earth would frequently reach 300 km/hr. Plants on land could not grow to any height above a few centimeters, and land animals would also be very constrained in size. In order to create our moon, that collision that ripped off our atmosphere at just the right time would have had to hit at just the right angle and with just the right amount of force. Given that a probability has already been assigned to any such collision, the additional criterion may be only around one thousand to one, and the chances of large land-dwelling animals reduce to one in a trillion.

Once all of the niches on land have been occupied, changes will be infrequent. Only major extinction events can jar the status quo sufficiently violently that new advances are possible. How likely are these extinction events? Earth has had about ten, of which five are labeled as the “Big Five” extinctions. Attempts to determine their causes have been thwarted by problems with interpreting evidence from so long ago.

Extinctions appear to have originated from three important causes: extreme volcanism, sea level decline, and asteroid impact. Volcanic eruptions are quite common, geologically speaking, on planets with plate tectonics. Mars no longer has moving continental plates because its crust has frozen to too great a depth. The planets that can have lasting life probably have plenty of volcanism and will occasionally have enough to cause extinctions like the one at the Permian-Triassic border. Sea level decline was probably the cause of the Ordovician-Silurian extinction about 440 million years ago when extreme glaciation took water from the seas. The ultimate cause may have been continental movement, a rare event.

The asteroid impact that ended the dinosaurs also cannot happen often and has only happened once with any certainty since the Cambrian explosion. Had this event occurred 200 million years earlier, dinosaurs would still rule the Earth, and 200 million years is only 5% of the Earth’s lifetime. It happened at a very auspicious moment in the history of life on Earth.

It’s very hard to assign probabilities for extinction events and harder to estimate which events pushed life toward intelligence. We know that one of the “Big Five” extinctions was very rare and some others may have been too. These extinctions occurred at an average of 100 million-year intervals, but without the rare ones, that interval may be closer to 200 million years.

Given enough time, a planet may have a sufficient number of extinctions to allow an intelligent branch of the evolutionary tree to develop. It may also have an extinction just as that branch begins and so destroy that future potential for hundreds of millions of year. Besides, there’s no evolutionary imperative for intelligent life, the sort we are that uses tools and builds on past experience to reach toward an unlimited future.

All evidence suggests that fortune smiled upon our planet (or misfortune if you are a species that mankind destroyed). We reached intelligent life in just 4.5 billion years. The chances are strong that most life-bearing planets would take much longer. There’s also a remote possibility that some may take less time. Without more information, I’ll give the probability of life evolving to intelligence by now odds of one in ten.

If you accept this reasoning, then you expect that no more than one in ten trillion stars have life on them and that it could be even 100 times fewer. In terms of galaxies harboring life, this result means that one galaxy in 100 has intelligent life.

What does this mean for SETI?

Does SETI Make Sense? Part I: Numbers
Does SETI Make Sense? Part II: Life
Does SETI Make Sense? Part IV: Communicating

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