The Drake Equation: Life

This is about Drake-equation factor fl, the number of Earthlike planets where life emerges, even very simple microbes.

The origin of life continues to be an unsolved problem. But research has approached that issue from two directions:

• Bottom-up: prebiotic synthesis
• Top-down: looking back in biological evolution

Prebiotic-synthesis experiments have made a variety of building blocks of organisms, like amino acids, the building blocks of proteins, and nucleobases, parts of nucleic acids (DNA and RNA). Robert Freitas in his book Xenology has a big list of work on this subject up to 1975: 7.3.1 – Prebiotic Synthesis. However, it is difficult to make another part of nucleic acids: ribose (Prebiotic ribose synthesis: A critical analysis – Springer). Gunter Wächtershäuser has proposed that a sort of prebiotic metabolic network had emerged in hydrothermal vents and similar places, but that notion has only limited experimental support (Iron–sulfur world hypothesis, Stirring The Volcanic Pot For A Hydrothermal Origin Of Life).

Going in the other direction, we seem to have a difficulty. The last universal ancestor of all well-studied present-day organisms turns out to be rather complicated. We can plausibly identify these features of it:

• DNA genome
• DNA-to-RNA transcription and RNA-to-protein translation
• The 20 canonical protein-forming amino acids
• ATP used as an energy intermediate
• Lipid-bilayer cell membrane
• Chemiosmotic energy metabolism: pumping hydrogen ions across the cell membrane out of the cell, and making them assemble ATP when they return
• Electron-transfer energy metabolism, ending in nitrogen oxides instead of in oxygen (Heme-copper oxidase superfamily)
• Carbon fixation, incorporating the carbon from CO2 into its molecules
• Combining CO2 and H2 for energy, giving methane or acetic acid
• Numerous protein enzymes, including a complete set of biosynthesis enzymes, likely making the organism autotrophic, able to make all its biological molecules

Fermentation was likely not ancestral, because of its complexity (How did LUCA make a living? Chemiosmosis in the origin of life).

But one can reconstruct some of the evolution of its ancestors, and the earliest sort of organisms that can be reconstructed with any confidence is the RNA world where RNA served as both information storage and as enzymes. Proteins were later developed to serve as enzymes, and DNA was later developed as a modification of RNA for master-copy duty. There are even bits of RNA in various coenzymes, relatively small molecules that work with enzymes.

The main criticism I’ve seen of the RNA world is the origin of the RNA. It’s difficult to make ribose prebiotically, and I’ve seen speculations about alternatives, like amino acids (peptide nucleic acids) and polycyclic aromatic hydrocarbons (PAH nucleic acids).

Finally, I must note that all well-studied organisms have several shared molecular-biological features, something that suggests that only one origin of life on our planet has had present-day descendants. It also suggests a lack of contamination by organisms from elsewhere in the Universe.

So the two research frontiers have yet to meet, and fl is still up in the air.