Astrobiology and the Origins of Life

by Dr. Stanley Awramik

Narrative Index

Astrobiology is a new field that discusses the origins, the distribution and the future of life in the universe. Two breakthroughs of importance to astrobiology are evidence for the possibility of the existence of life in the past on Mars and the discovery of planets around nearby stars. Speculation and early experiments on the origins of life from Charles Darwin, Alexander Oparin, H.B.S. Haldane contributed to a breakthrough experiment by Stanley Miller. Miller took non-living material that was on the early Earth--hydrogen, methane, ammonia, and water--provided an energy source and produced amino acids. In the 44 years since Miller's experiment, scientists such as Juan Oró and Tom Cech have produced nucleic acid bases and riboenzymes but no self-replicating life or organism or cell has yet been produced in the laboratory.

What do we know about the timeline for origins of life on Earth? Earth formed from the accretion of planetesimals about 4.6 billion years ago. Current estimates are that it took 200 million years, or to about 4.4 billion years ago, until the Earth had a solid crust, had water and was a habitable environment for life. The oldest rocks are now placed at about 3.96 billion years and the oldest fossils are from 3.5 billion years. Four hundred million years of geological record are missing. This could be the origin of life window.

The oldest fossils are about 3.5 billion years; however, in rocks dated 3.8 billion years, analyses of isotopes of carbon suggest that carbon fixation and maybe even photosynthesis was around then. So now we have evidence of life at 3.8 billion years ago, some rocks at 3.96 billion and habitability maybe as far back as 4.4 billion. Think of the primitive Earth crust-- heat, volcanic activity, radioactivity, energy from impacts of asteroids and comets, water-- as just one big chemical evolution experiment.

The fossils from 3.5 billion years are thought to be the remains of Cyanobacteria. Microbiologists and molecular biologists have concluded that Cyanobacteria were one of the last major groups of bacteria to evolve. That would suggest that much of the microbial evolution at the phylum or division level may have already been around by 3.5 billion years. If 3.8 billion years is the date of the last common ancestor that leaves 300 million years for life to evolve to the level of Cyanobacteria. Microbiologists, however, believe 300 million years is too short a period of time for that level of evolution to have occurred.

So where does that put the origin of life? Let's go back to that exciting breakthrough mentioned at the start of this talk--the alleged microfossils found in the Martian meteorite in Antarctica dated 3.6 billion years. The fossil tubular structures are on the order of 40 or 50 nanometers in diameter. Based on our understanding of genetic systems on Earth, we're not sure if a cell that small could contain enough genetic material to exist.

We are left with questions and the understanding that there is a need for a cosmological perspective when exploring the origins of life. This brings us to the new field of astrobiology.