This is Part 2 of our short series, “Abiogenesis and the Origin of Life”. Read Part 1 HERE.
How do scientists think life began?
First, it must be noted that like many aspects of scientific knowledge, understanding the processes and mechanisms of abiogenesis is far from complete. Research is ongoing, and there is lively debate about the specifics of certain hypotheses concerning the origin of life. However, the scientific community of biologists and biochemists as a whole accepts a number of facts, some to a farther extent than others, and the purpose of this article is to illustrate some of these facts and interpretations.
It is often assumed that the prebiotic (that is, before the first cell) environment was rich in organic materials that life eventually co-opted for its own purposes. Through research such as the Miller-Urey experiment and its recent re-analysis, there is evidence that the prebiotic environment contained a complex mixture of organic compounds, including nucleic acids, the building blocks of DNA and RNA, amino acids, the building blocks of protein, and phospholipids, the molecules that compose cellular membranes.
Phospholipids are particularly important molecules in the proposed models of abiogenesis. These molecules have the ability to spontaneously form bilayers in aqueous solutions. Cellular membranes in cells living today are composed mostly of these so-called lipid bilayers. However, bilayers within a large volume of water do not typically cover a flat area; instead, they will form tiny spheres called vesicles. Vesicles have the ability to grow by coming into contact with smaller spheres composed of fatty acids, called micelles. These phospholipids and the structures they form are important – they provide a separation of environments between internal and external, while also allowing simple molecules to pass through.
A simplified model of abiogenesis proceeds as follows:
- Some sequences of nucleic acids have the ability to self-replicate; that is, these sequences are able to catalyze (speed up) the reactions that enable them to make duplicates of themselves.
- A self-replicating nucleic acid within a vesicle would have ample resources to produce more self-replicating nucleic acids.
- As more nucleic acids replicated, producing even more copies, the vesicle would grow until mechanical forces divided it, and it would then be able to reform spontaneously into a number of smaller vesicles containing fewer copies of the nucleic acid.
- Although these self-replicating molecules had the ability to copy themselves, much like modern cells this ability was imperfect – chance copy errors would provide the newly separated “protocells” with sequence diversity.
- It is thought that this form of replication, combined with copy error, continued over a relatively long period of time, and inevitably lead to the natural selection of useful features of the genome, including the ability to encode for genomic function with genes and their regulatory sequences.
- Once the functions considered necessary for life have been encoded, the system of self-replicating nucleic acids and phospholipids is considered to be alive, and abiogenesis is complete.
From there, evolution continues to take its course, and the diversity of life engulfs the planet for millions of years until one day, a species will evolve with the mental capacity to ask the question, “How did life begin?”
References and Further Reading:
http://www.livescience.com/13363-7-theories-origin-life.html
http://www.space.com/3511-enduring-mystery-life-origin.html
http://www.sciencedaily.com/releases/2011/06/110609174852.htm
http://en.wikipedia.org/wiki/Abiogenesis