The next question on the journey from the Big Bang to you and me concerns whether multicellular life will inevitably evolve. First, multicellularity on Earth requires an oxygenated atmosphere. Oxygen will be a waste product of most forms of early life that use chemosynthesis and photosynthesis to power metabolism. It is also extremely reactive, and free oxygen cannot accumulate in the atmosphere until elements or molecules it easily reacts with have been oxidized. Planets are large, and the oxidization of surface molecules takes time. On Earth it took about 1.5 billion years before atmospheric oxygen began to increase.
Multicellular life required atmospheric oxygen to evolve, but once oxygen levels started to rise it evolved at least twenty-five times. Eukaryotes needed mitochondria to use the oxygen to run their metabolism. Mitochondria evolved from bacteria that were able to survive inside other cells. Predation or parasitism had to evolve to allow these bacteria to enter other cells before they were co-opted into a mutualism by the host. Over time, these bacteria evolved into organelles such as mitochondria, that are used by all multicellular organisms to run their metabolism. Evolution is very good at producing organisms that can exploit abundant, under-utilized resources, so it is possible that predation and parasitism would evolve given enough time on other planets. It also seems probable that once predation and parasitism evolved, mutualisms with bacteria living inside host cells could also arise. All eukaryote cells contain mitochondria, derived from a mutualism with aerobic bacteria, while plant cells also contain chloroplasts that evolved from photosynthetic bacteria such as the cyanobacteria. Given such mutualisms evolved independently at least twice on Earth, it seems plausible that they could evolve elsewhere too. My hunch is that multicellular life will almost inevitably evolve from simpler life forms given sufficient time.
On Earth, a lead time of over 2 billion years was required before multicellular life evolved. Our planet had to remain habitable while avoiding cataclysmic extinction events such as local supernovae, meteor impacts, or the loss of atmosphere from solar winds that could wipe out all life. Earth did this, but we currently have no idea whether this is typical or not. It seems likely that once life evolves, it inevitably becomes more complicated, but life is fragile, and is permanently at risk of being killed off by violent cosmic events. We exist because Earth remained habitable for 4 billion years, allowing evolution to work wonders, but we do not know whether such durations within habitable zones are commonplace or rare.
Once complex multicellular life had evolved on Earth, there was no guarantee that humans would appear a little over half a billion years later. Random, unpredictable events driven by mutations mean that evolution is unpredictable. In an experiment with the bacterium E. coli, Rich Lenski allowed twelve colonies of bacteria to adapt to the same environmental conditions. At the time of writing, the experiment had been running for 75,000 generations. Initially it looked as if each population was evolving in a phenotypically similar manner, but examination of the genomes of the bacteria revealed that different colonies accrued different mutations. In other words, although phenotypic adaptations initially looked similar, they were not achieved via the same genetic mutations. Such a pattern is not unusual – there are often multiple genetic routes to the same phenotypic solution.
The most remarkable adaptation occurred in one of the twelve colonies shortly after generation 31,000. In the presence of oxygen, E. coli use glucose as a primary source of energy to run their metabolism, but they can also run their metabolism off a compound called citrate in the absence of oxygen. E. coli in the laboratory are typically fed a diet of glucose and citrate, but will not utilize the citrate in the presence of oxygen, and this was the diet that was fed during Lenski’s long-term evolution experiment. One colony evolved to utilize citrate in the presence of oxygen. It is difficult for E. coli to do this because it involves a series of mutations and not just a single one. These mutations occurred only in one colony. The bacteria in this colony can now use a resource unavailable to the other E. coli colonies. Over longer periods, phenotypic evolution is not repeatable, and there is no guarantee a particular life form will evolve.
Such rare mutational events, perhaps driven by quantum fluctuations, will have played occasional but important roles in the evolution of life on Earth. It is highly improbable that life on other planets would experience the same set and order of random mutation events as occurred on Earth, particularly if it uses different triplets to code for amino acids. Scientists do not know enough about the genetic architecture of most phenotypic traits, or about which phenotypic traits are most likely to arise, and which are less likely to evolve. Life on Earth today is a combination of what has gone before in terms of mutation and selection. Although natural selection can frequently be repeatable, genetic mutation is less repeatable, and this can lead to quite different outcomes over millions of years. Deer and kangaroos have similar ecological roles respectively in Europe and Australia, and they are both capable of thriving on vegetation, but they look very different. The evolution of multicellular life on other planets may result in some outcomes that look familiar to us, but others that might look very odd. Aliens really could be little green men.
Once multicellular life arose, it seems inevitable that evolution would find ways to detect light, sound waves, aromatic molecules and touch, and this would require a central nervous system and some form of brain. Consciousness would then naturally emerge, and in time intelligence would appear. Intelligent life arose on Earth in the form of humans, and their close cousins, the Neanderthals. Other, less closely related species, including dolphins, elephants, crows and octopuses, stand out as also being able to solve quite complex tasks. Although complex language and technological development have evolved only in primates, this does not preclude it from evolving in other species.
Several writers and scientists have concluded that life, and intelligent life in particular, are likely to be rare. I don’t believe we can make that call yet, as we just do not have enough evidence. Most things in our universe, be they galaxies, stars or star systems, have distributions. For example, galaxies may contain a thousand stars or as many as 100 trillion. Their size varies enormously, with the Milky Way lying somewhere in the middle of the distribution. In the absence of evidence, the most sensible thing to conclude is that what you are observing is more likely to be average than not. Our galaxy is average, the sun is average, and rocky planets in the habitable zone are also not at all unusual. It may turn out that Earth is quite typical and that life is abundant. We will not find it on every planet, or even around every star, but if I were forced to guess, I would say that life evolves frequently on rocky planets with liquid water and volcanic activity, and that we are far from the only planet where intelligent life has emerged. Life is chemistry, and chemistry follows sets of rules, so as long as conditions are right, life will emerge.
As a species, humans are very probably unique in the universe. I cannot see how species with the same genetic code as ours evolved to live on planets around other suns. Genetic mutations occur at random. We are a product of genetic mutations raining down on our ancestors’ genomes for 4 billion years. An identical sequence of random events will not have happened on another planet, anywhere, so as a species we will be unique.