The membranes that life uses are made of a type of molecule called a lipid, and lipids are made up, in part, of smaller components called fatty acids. You probably won’t be surprised that fatty acids have been found on meteorites, including the one that hit the ground near Murchison. Like all other key components of life, these essential molecules are not restricted to Earth, forming elsewhere in the solar system too.
The fatty acid molecules are long chains built from carbon, hydrogen and oxygen atoms. In cell membranes, two fatty acids are joined with a molecule of a compound called glycerol and are then attached to a phosphate molecule. Phospholipids, as the resulting molecules are called, look a bit like a tadpole with two tails. The head is the phosphate molecule, and the tails are the fatty acids. When placed in water, these phospholipids spontaneously form two joined layers, a so-called lipid bilayer, with the heads on the outsides of each layer, and the fatty acid tails pointing inwards towards one another. These bilayers form because phospholipids will not dissolve in water. Instead, different parts of the molecule have different electric charges, which means they interact with water molecules and with one another to form a bilayer. The phosphorus ions in the head of the molecules are attracted to the negative-charged parts of the water molecules. In contrast, the fatty acid tails are hydrophobic – meaning they are repelled by water. These properties, stemming from the electromagnetic force, mean that the molecules rapidly form bilayers in water as their heads and tails are respectively attracted to and repulsed by the water molecules. It is easy to form phospholipids. When placed in water, these lipids spontaneously form membranes, and it is these lipid bilayers that are the basis of membranes that surround cells.
Once a lipid bilayer forms it acts as a barrier to ions, proteins and other molecules. Such molecules cannot cross the bilayer as it acts like a fence around a field that keeps sheep trapped inside and wildlife out. One challenge early life would have faced was how to move molecules across the bilayer, and many researchers argue for leaky membranes. Modern life solves it with proteins embedded in the membranes that surround cells, as in the mitochondria example described earlier. These embedded proteins act a bit like gates in my field-of-sheep analogy.
Exactly how autocatalytic reactions became embedded within the first cell, or how the first cells divided, is unclear. Nonetheless, the necessary molecules for life were abundant on the early Earth, and once autocatalytic reactions began they would have spread. Membranes form easily, so the key component of a replicating cell would have been available for chemistry to work its magic. But there is still one thing we have not touched on. Where did the energy to run these reactions come from, and how did life use it? This is where volcanic activity comes to the fore, potentially driving the metabolism that early life would have used.
Life today is powered either by energy from light, or by energy from highly reactive molecules such as hydrogen sulfide or, in the case of animals, by energy gained from consuming other forms of life. The chemical reactions involved in metabolism differ depending upon the source of energy, but the result is always the same: the creation of molecules of ATP. ATP is life’s fuel. It is reasonably stable but can be coerced to release energy easily enough, and it does not blow up. It is a much better chemical to power the germ cells and the disposable soma than azidoazide azide.
Life needs energy to run, but in doing so also gives off energy. That is why you are warm. The travails of staying alive require you to burn energy. Light is extremely abundant on Earth, and anywhere close to stars, and the cyanobacteria that formed the first stromatolites ran their metabolism using light. However, scientists think that the first living organisms used chemosynthesis to power their replication. They used reactive chemicals, and a great source of these would have been volcanic vents in the ocean or on land. Volcanoes produce large amounts of compounds that readily share, donate or steal electrons from other compounds. In places like Yellowstone, bacteria living in sulphur-rich hot pools run their metabolism from hydrogen, hydrogen sulfide and carbon dioxide, and that is a good candidate suite of chemicals to power the first cells.
The use of reactive chemicals by organisms to power metabolism is known as chemosynthesis. Modern-day bacteria use a range of chemicals, but each produces a sugar from a carbon-containing molecule such as carbon dioxide or methane gas. Some chemosynthetic bacteria use hydrogen to power the production of sugar, while others use ammonia or hydrogen sulfide. Organisms that require oxygen to live use a different type of metabolism called the Krebs cycle, or citric acid cycle, which you might remember from school. Although involving more reactions than most forms of chemosynthesis to create sugars, key components of the Krebs cycle naturally emerge in the right mix of chemicals. Like the formation of cell membranes, various forms of metabolism naturally occur under the right conditions.
In the next chapter, we will meet an ancient primitive organism called LUCA from which everything on Earth today is descended. Although long extinct, scientists have had a stab at reconstructing its genetic code, and this reconstruction points to genes involved in the chemosynthesis of hydrogen sulfide or similarly reactive chemicals. I don’t know whether the first cell used ATP, but LUCA may well have done, suggesting that the universal fuel of life today was adopted early. Scientists have yet to find ATP on meteorites, but they have found the separate adenine and triphosphate building blocks. Like other key molecules of life, its building blocks can be made elsewhere in the solar system before being delivered to Earth. Emergence-of-life chemists have also made compelling arguments that proton pumps have been central to life from its earliest days, so ATP may have been used surprisingly early.
The details of exactly how and where life began are unknown, and there is still a lot to discover, but scientists have made much progress. We know why life emerged – like all chemical reactions, it happened because it was energetically the easiest option. Given the right mix of chemicals and energy inputs, life is the favoured outcome. We also have some idea what had to happen for life to start. Scientists know that autocatalytic reactions fuelled by a metabolism run on chemicals like hydrogen sulfide became enclosed in a membrane, and that this happened in a solution rich in organic compounds both seeded from space and produced on Earth. As early life continued to develop, driven by errors in replication, the chemistry inside the membrane became more complicated than on its outside. We also know that early life spread because the exponential growth of a molecule is a powerful force. Competition led to increases in complexity until the first protocell existed, but we do not know the details of each step. In time, DNA and the genetic code life uses today became established, and, from then on, life did not look back. There is a huge divide between the simple chemistry we conduct in labs and chemistry of even the simplest life, but we have a roadmap of what had to happen, and we know that the key molecules would have been available on the early Earth. What we don’t know is how it all happened, and what the right mix of chemicals is, how much variation there might be in that mix, or how long it might take. For this reason, the emergence of life is still shrouded in mystery, but the shroud is steadily being lifted, and in the next decade or two I suspect life’s emergence will become increasingly better understood, and may even be replicated in the lab.