In three previous posts (part 1, part 2 & part 3) I summarized the key highlights of the history of life on Earth, beginning with the Earth’s creation (hint, not by God), and ending with humans’ inventing the radio. I described the most important events, but I didn’t spend any time looking at their implications, particularly what this means for the search for alien life.
What can the history of life on Earth can tell us life on other planets?
Well I’d say it could teach us two major things:
1. Life may be common in the Universe, particularly on planets like the Earth
The fossil and biological evidence suggests that life likely emerged quickly on the Earth. In the first post in the series I put this at 3.8 billion years ago or at 04:00 on the 24-hour clock (if we say that the Earth has existed for 24-hours). This is perhaps a relatively conservative estimate, and some scientists believe life appeared even earlier. Whatever the exact time, it seems that life emerged on Earth almost as soon as the Earth could support it. Before this time asteroids and comets were bombarding the Earth frequently, and much of its surface was volcanic. Life may have tried to get started but was likely quickly annihilated by these hostile conditions.
If life appeared on Earth as soon as it could have, then this may indicate that life is very likely to develop in other locations too, if the conditions are suitable i.e. a supply of carbon, energy and liquid water. Carbon and the other key elements are relatively common in the Universe, and stars’ provide energy, thus if planets like Earth exist at the right distance from their stars, they may well have life. A great way to test this hypothesis is by looking for evidence of life on Mars, as early in its history the red planet was probably similar to Earth, with an atmosphere, a warmer climate and even surface liquid water. Mars is currently being studied by NASA and ESA, fingers crossed they find some fossils!
2. Intelligent life may not be very common in the Universe, particularly life capable of interstellar communication
Although life appeared on Earth relatively quickly, intelligent life in the form of human beings took a very long time to evolve. If we use the 24-hour clock analogy, life first emerged at 04:00 but human beings didn’t emerge until, 23:59:56, we’ve only been around for roughly 4 seconds!
It gets worse for the alien hunters. Life on Earth remained single celled (like bacteria) for well over half of its history, from 04:00 to 17:40 on the 24-hour clock, at which point the first multicellular life emerged. The first true animals didn’t emerge until 20:56 in the Earth’s evening, and the first four legged land animals didn’t appear until 21:55. The kind of advanced life we take for granted today evolved relatively late in the Earth’s history. So does this mean that advanced life will probably emerge on other Earth-like planets, but will take a long time about it?
Maybe not, it gets even worse. Geological evidence suggests that many of the key steps in the evolution of life happened after enormous extinctions. Multicellular life and true animals may both have evolved after incidents in which the entire Earth froze over like a giant snowball, mammals didn’t evolve until after potentially the greatest of mass extinctions (the Permian mass extinction), and remained as little more than small rodents for hundreds of millions of years until a chance meteorite strike wiped-out the dinosaurs. This suggests that evolution does not inevitably lead to intelligent life, evolution may require regular extreme-shocks to move it along. Think about it, humans probably wouldn’t exist if the dinosaurs hadn’t gone extinct.
All this means that just because life can become complex and intelligent doesn’t mean that it necessarily will do. If many Earth-like planets don’t receive the right fortuitous sequence of catastrophes’, then maybe life will remain relatively simple for the whole of a planet’s existence.
And it gets even worse for the search for intelligent aliens. Humans’ have only been around for an incredibly short time, and we’ve only had the radio for little over a century, a few tens of thousands of seconds of the 24-hours of Earth’s existence. Furthermore we don’t know how long we’ll be around for. We tend to think of ourselves as masters of the Earth, we’re intelligent, we can build cars and iphones, and we can change the world around us to better suit our needs. But we also have the capability to destroy most life on Earth, and we don’t really know how well our civilisation can withstand the worst that the Earth has to throw at us. I guess we’ll get a taster with climate change. Who knows how long we’ll be around before we go extinct? (All species go extinct eventually). Maybe we’ll be short-lived species?
So, to sum up, although life may be common in our Galaxy, intelligent life capable of receiving and sending radio waves may be very rare, and may only exist for a short-time before it, or something else, destroys it.
Earlier I wrote about the Drake Equation, which is an attempt to predict the number of advanced civilizations in our galaxy, and about the Fermi Paradox, which asks why haven’t we found intelligent life in the Milky Way if there are so many places in which life can evolve and so much time? We don’t know the answer to either, but I think this is a pretty good explanation; there may be loads of alien life, it just can’t use a radio.