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Evidence for Abiogenesis

Abiogenesis (a-bio-genesis): The process by which living beings arise from non-living components.

One of the tenets of modern biology is the idea that cells arise only from existing cells. Living beings, no matter how simple or small, do not spontaneously appear from their surroundings. Instead, the process of cell division and replication allows certain types of existing cells to create new cells. We see this everywhere in the world, from the growth of bacteria cultures to human reproduction.

Where, then, did the first cells come from? When in Earth's history did the first cells appear? How did they come from the biological components (such as proteins and RNA) that we discussed on previous pages, and how did those components come to exist on Earth?

Cells differ in their structure and makeup, but at a minimum, all living cells have the following components:

Let's examine how each of these may have arisen from non-living components.

Cytoplasm

This is perhaps the easiest component to explain. All theories of early life involve the presence of water, which would have included salts and organic molecules. The cell membrane would necessarily have enclosed some of this water.

Later, the process of evolution would allow cells to become more complex, adding components that would help them survive longer or replicate themselves.

Cell Membranes

Many different structures may have allowed the first cell membranes to form. Almost any small, naturally-occurring bubble shape might have provided the surface on which the membrane might have formed. Examples include the bubbles in seafoam, water droplets sprayed by geysers, bubbles in soft clay, and the bubbles that form in volcanic rocks like pumice.

The video series on the From Chemicals to the First Cells page shows the process of this formation very nicely.

Genetic Material

This is the most complex part of early cells. Without genetic material, the cells cannot reproduce or evolve.

You have already learned a lot about proteins and RNA, and how they are the building blocks of life. One of the reasons that these large molecules are so important is that they can accomplish many functions. RNA in particular is very versatile: it can act as a carrier of information, as the source of that information, and as a catalyst that encourages certain reactions. With these functions and its versatility, RNA is considered a good candidate as the original source of genetic information in early cells. You can read more about this on our page about the RNA

RNA is assembled from nucleobases (on our world, specifically adenine, guanine, uracil and cytosine). Each of these is a chemical compound of moderate complexity, similar to an amino acid. If we can see how those were created from raw materials, the transition from them to the more complex proteins and RNA is an easy step. Can we create amino acids from the most basic building blocks that would appear on a lifeless world?

Luckily, there has been an experiment performed to answer that exact question. In 1953, Stanley Miller and Harold Urey created an environment in which water vapor, methane, ammonia, and hydrogen gas were mixed. All of these gases are known to exist on other planets in our own solar system, as well as in interstellar clouds. This mixture of gases was exposed to electrical discharges similar to lightning bolts, though much smaller. The entire apparatus was sterilized and separated from outside contamination. The Miller-Urey experiment revealed that just a few weeks of this process was able to create over half of the amino acids needed for life on Earth, as well as sugars and other hydrocarbons.

Other experiments have verified that different mixtures of gases can also create amino acids when subjected to electrical discharges, that volcanic environments as are just as effective as electrical discharges, and that the original experiment can produce even more amino acids than originally thought.

How Exactly Did It Happen?

This is an area of science in which we have many well-supported hypotheses, but no definite answers. There are several arguments that discuss how life may have arisen on Earth billions of years ago. We may be able to test these to see which way is fastest, most efficient, and most reliable. However, because most of Earth's earliest rocks and fossils have been recycled in the planet's mantle, we have lost the evidence that could tell us which theory is the way that life actually arose. It is even possible that life arose several times, being wiped out by local or global disasters (such as asteroid impacts) before finally catching on well enough to survive.

Солнечная система и ее тайны