10.2: Cells and self-replicating systems evolved together to create the first life.

After the generation of numerous organic molecules such as amino acids, the second phase in the generation of life from non-life was probably the assembly of these building block molecules into self-replicating, information-containing molecules. This is where things get a bit more speculative. It’s complicated enough to generate a complex organic molecule, but it’s a whole lot more complicated to generate an organic molecule that can replicate itself. Researchers believe that to get to the replication phase, enzymes, or something with the catalytic activity of enzymes, were required.

Phase 2: The formation of self-replicating, information-containing molecules. Recently, researchers discovered that the nucleic acid RNA can do what proteins do—catalyze reactions necessary for replication—meaning that this single, relatively simple molecule could have been a self-replicating (i.e., able to make copies of itself) system and a precursor to cellular life.

These self-replicating molecules raise an important question: When exactly was the threshold between living and non-living crossed? It has been proposed that in the early world, self-replicating nucleic acid molecules carried the information on how to replicate and served as the machinery to actually carry out the replication. Is that enough? At this point, it is reasonable to ask, again: what exactly is “life”?

Fossils of 3.4-billion-year-old cells have been found in rocks in South Africa and Australia. These cells appear to be prokaryotic cells, similar to living bacterial cells, with no nucleus, no organelles, and a circular strand of genetic information (FIGURE 10-3). Some even look as if they are in the process of dividing. Although, superficially, the fossilized cells look like just a couple of circles, several lines of evidence support the idea that they are indeed remnants of cells. These include: (1) the age of the rocks themselves has been reliably determined, (2) the size of the circles is similar to that of modern-day prokaryotes, and (3) the ratio of two carbon isotopes (¹²C/¹³C) is more characteristic of fossilized organisms than of typical rocks that do not contain fossils. But many questions remain, including: Were these cells the first living organisms on earth? And were they descendants of earlier, self-replicating molecules of RNA? Because no earlier fossils have been found, it is difficult to answer these questions.

Earlier, we mentioned that there are characteristics shared by all living organisms and living systems and that these include the ability to replicate and the ability to carry out some sort of metabolism. So the self-replicating RNA molecules were right on the border; they were able to replicate, but not able to carry out metabolism. We now explore the critical third phase in the generation of life from non-life: the development of a membrane that separated these self-replicating small molecules from their surroundings, thus forming cells and facilitating metabolic activity.

Phase 3: The development of a membrane, enabling metabolism and creating the first cells. One way that self-replicating molecules could have acquired chemicals and used them in the controlled reactions of metabolism was by packaging the molecules within membranes. As we saw in Chapter 3, cell membranes are semi-permeable barriers that separate the inside of cells from their external environment. Membranes make numerous aspects of metabolism possible. In particular, they make it possible for chemicals inside the cell to be at higher concentrations than they are outside the cell. Differences in chemical concentrations inside and outside a cell are essential to most life-supporting reactions.

So, if we could combine a self-replicating molecule and some metabolic chemicals into a unit, surrounded by a membrane, life would be possible. But how could this have happened initially? Some evidence suggests that the first cells may simply have formed spontaneously. Specifically, researchers have found that mixtures of phospholipids placed in water or salt solutions tend to spontaneously form small spherical units that resemble living cells. These units may even “sprout” new buds at their surface, appearing to divide. Because these cell-like units don’t have any genetic material, however, they cannot be considered to be alive. But if, at some point, units like these incorporated some self-replicating molecules inside, maybe by forming around them, such microspheres might have been important in the third phase in the generation of life from non-life: the compartmentalization of self-replicating, information-containing molecules into cells. If this did occur, the final step in the creation of something from nothing would be complete (FIGURE 10-4).

The exact process by which life on earth originated is still uncertain, and research continues. What is clear, however, is that life now abounds in great diversity. In this chapter we explore that diversity and how it is generated. In doing so, we’ll investigate what a species is and consider how individual species split and create additional species.