Professor Gary Ruvkun (Harvard) "The Hallmarks of the Last Universal Common Ancestor and where to look for it today"

Abstract

Ever since the universality of DNA and the genetic code were discovered between 1953 and 1965,
molecular biologists and chemists have explored the steps required to evolve life as know it from simple
chemistry. As the first genome sequences from a few bacteria, fungi, plants and animals emerged around
2000, a set of about 500 RNA and protein-coding genes were discovered to be universal across the entire Tree
of Life, from bacteria to archaea to eukaryotes. the genes of the last universal common ancestor (LUCA) to all
life on Earth. Bacteria-like fossils of LUCA microbes have been observed in 3.5 billion year old strata, giving
the tree a time stamp. But if LUCA had already evolved 3.5 billion years ago, LUCA only had about 500 million
years to evolve from the simple chemistry to its very complex genome. And LUCA was not primitive--its 500
genes are an evolutionary pinnacle that nearly all life on Earth still depends on. An alternative hypothesis of
“panspermia” posits that life has been spreading across the universe since long before the birth of Earth, either
by random exchange of living organisms between planetary systems (science), or by the directed insemination
of particular planets (Science Fiction). An often repeated critique of panspermia is that migration of life
between planets simply places the problem of the evolution of life to chemistry on another planet; it does not
help solve the problem of evolution of life. This omits the crucial dimension of time: the Milky Way is much
older than the Solar System, 13.5 billion years vs 4.5 billion years. This extra 9 billion years is not just the
factor of 2 or 3 compared to the age of the Earth. There is a 100x time difference between the standard
astrobiology theory – that life evolved on Earth in a few hundred million years from the primordial soup to the
very highly evolved DNA world as the Earth cooled 4.1 billion years ago vs. the 9 billion year period allowed for
the evolution elsewhere in the galaxy, and then seeding of highly evolved communities of microbes to the early
Earth as it cooled. A key prediction from such a Spread of Life across the Galaxy model that many of the 500
genes we now know are common to life on Earth would also be shared with life on habitable planets elsewhere
in the Milky Way (for example SETI signal searches should be for the 16S rRNA sequence, not
3.14159265359). The narrowest test of panspermia is between Earth and Mars, the most plausible planet of
the Solar System besides Earth to have life, and easiest to sample for LUCA genes. Maria Zuber and I have
directed such a long term project with Chris Carr, a joint scientist in our labs, funded by NASA. But other
approaches that do not depend on Mars lander instrument development are to seek DNA in meteorites from
Mars that have transferred to Earth without massive heating from their collision-based ejection from Mars or
reentry into the atmosphere of Earth.