My conclusion for how life emerged after reading extensively on the topic is that pools rich in amino acids, phosphates, lipids, nucleobases and other key building blocks of life formed in freshwater environments in hydrothermal fields, although I cannot rule out life emerging in the oceans. Hot water temperatures, coupled with a steady influx of chemicals from volcanic activity, resulted in stable, complex, organic compounds such as DNA, RNA, phospholipids, proteins and ATP forming. These large and complex molecules formed because they were energetically favoured, being the stable state of the system. The structure of some of these molecules facilitated metabolism and replication. Life assembled itself, with its autocatalytic abilities and complex structure emerging from a rich soup of complicated organic molecules that were themselves formed from a concentrated pool of simpler chemical building blocks either forged on Earth or delivered to our planet on meteorites. The problem in testing this hypothesis is we don’t know enough about the environment in which life emerged.
If scientists were to simply take random mixes of chemicals and subject them to the environment thought to exist on the early Earth it may take millions of years before life arises in a test tube. Although life emerged quite quickly on the early Earth, it is only quick on the scale of the 4.5 billion years of Earth’s history. It may have taken millions, or even hundreds of millions, of years to appear. Fortunately, chemists can do much more than randomly mix chemicals together. Through the study of meteoroids and meteorites they are developing an understanding of which building blocks were likely available to early life. Computer simulations of the behaviour of these compounds built from our understanding of the workings of electromagnetic force are playing an increasingly important role in emergence-of-life research, and it strikes me that identifying how life emerged is a problem that may be tractable for the latest artificial intelligence algorithms. AI has already helped gain insight into how chains of amino acids fold into proteins, a problem that humans had long found intractable. AI can be very effectively applied to help us understand complicated chemistry. The irony of using non-living artificial intelligence to help create the living is not lost on me. It would grab headlines around the world.
Once evolution had moulded the DNA and the genetic code that life uses today, things really took off, and life proliferated. Life has clung to Earth for nearly 4 billion years and has coped with everything the universe has thrown at it, from solar flares to meteor impacts to frigid temperatures. It has not just coped, it has thrived, and now it occupies nearly every crook and crevice of our planet, from the floors of the deepest oceans to the skies above the tallest mountains. The next part of our history focuses on how life spread and developed from its simple origins, and how it overcame some very significant challenges.
Life Conquers All
In the previous chapter I described how and why competition between autocatalytic molecules would have resulted in the evolution of replicating systems of chemicals in the environments where the first life formed. I doubt the description upset many readers. In contrast, when evolution is discussed with respect to more complex forms of life, and particularly humans, some people challenge its primacy in determining our existence. Yet the evolutionary processes that produced the first living organism are the same as those that produced humans nearly 4 billion years later. Evolution is as real as gravity, electromagnetism and the strong and weak nuclear forces, and without it we could not exist. Biologists understand evolution to about the same extent that physicists understand the strong and weak nuclear forces and chemists understand electromagnetism. They know how and why evolution creates new species. Evolutionary biologists like me have a clear definition of evolution and we know how to measure it. We understand how genetic differences arise and how natural selection operates on this variation to generate new forms of life. We understand how changing environments result in evolution, and in some cases biologists can predict how natural selection will change animals and plants. If you accept science as a way of finding out how and why the universe works as it does, you should accept that evolution is as real as the four fundamental physical forces discussed earlier.
There are two reasons why evolution can be a challenging topic. First, some people deny evolution because they find it hard to accept that humans evolved from simpler life forms such as single-celled bacteria and, more latterly, chimpanzees. Yet the comparisons of genomes between species as diverse as bacteria that cause disease, bananas, sea squirts, worms, fruit flies, dogs, gorillas and us provide overwhelming proof that all forms of life on Earth are related to one another. To generate this astonishing biological diversity, evolution needed billions of years, and during that time it invented death and sex, tiny structures in cells called mitochondria and chloroplasts, thousands of new types of molecules including forms of carbohydrate, protein and lipid, numerous cell types, and multicellular organisms such as the silver birch tree, Woofler and you. This chapter is about what had to happen for animal, plant and fungi species to become complicated. There are thought to be about 8.7 million species of plants and animals alive on Earth today, along with an unknown number of fungi species, and hundreds of millions of species of single-celled microbes. Evolution has invented a vast number of ways for organisms to make a living. We will encounter some of these as we think about what happened for the first mammals to evolve.
The second reason that evolution can be challenging is it is a quite complicated concept. Some readers will find it harder to understand than the workings of the four fundamental forces. Evolution is difficult because it involves several things happening at once. On their own, each of these things is easy to grasp, but when combined things can become complicated. I shall try to briefly summarize.
All organisms need resources such as food, water and shelter to live, and sexually reproducing species such as humans need to find a mate to reproduce. However, there are many things in the environment such as predators, or disease, or large numbers of competitors, that can make finding resources, including mates, difficult. Individuals that are good at finding, acquiring and using resources while avoiding the threats that the environment imposes upon them tend to have particular attributes such as the ability to deter things from eating them or the ability to mount an immune response to an infection. These attributes are determined, at least in part, by an individual’s genes – its DNA code. Those individuals that succeed in acquiring resources and avoiding threats tend to produce the most offspring, and this means their genes are represented in more individuals in the offspring generation compared to individuals who did not have these particular attributes. These genes are likely to produce the desirable attributes in offspring that made their parents successful. Evolution proceeds as genes that determine attributes that enable individuals to acquire resources while avoiding threats increase in frequency within a population. Over short periods of a few generations, evolution happens quite slowly and not a huge amount of change is observed. Over many hundreds of thousands or even millions of generations, populations can significantly genetically diverge, and new species can evolve. Evolution is consequently a process driven by which individuals are best at surviving and reproducing, but evolutionary change over a few generations is usually quantified by looking at how populations change over time or by comparing genetic differences between separate populations of the same species. On longer timescales, evolution is quantified by looking at genetic differences between individuals of different species. And there are a lot of species to compare.