The last common ancestor

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Woese CR 2000 Interpreting the universal phylogenetic tree. PNAS 97:8392

If we look at the 3 Domain phylogenetic tree,...

 

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... what can we infer about the last common ancestor?

It's probably fair to suggest that any features common to all three 'Domains' were inhereted from the last common ancestor, including:

  • DNA, the Universal code, and most genes
  • transcription and RNA polymerase
  • RNAs of all types (rRNA, tRNAs, etc)
  • translation and the translational apparatus
  • most proteins, and the metabolic pathways they generate
  • membrane and cellular structure

In other words - pretty much everything about the basic processes of life. Most biochemical evolution must have predated the last common ancestor. The phenotype of the last common ancestor was probably thermophilic, because all deep/primitive branches are thermophilic. The last common ancestor, if it existed as a single organism, was probably something like Thermococcus celer is today. Most of evolution since has been peripheral.

However, the last common ancestor may not really have been a single organism, but rather a population of organisms (like 'mitochondrial Eve') or a 'communal' organism. There is theoretical reason to believe that lateral gene transfer may have been pervasive prior to the last common ancestry, and the 3-way split between Bacteria, Archaea, and Eukarya may have been lifes first real experiment in geneological diversification.

Before the Last Common Ancestor?

So, if most of evolution (at least biochemical evolution) predates the last common ancestor, let's have an expanded look at the the tree:

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The "progenotes" in the figure above refer to organisms in which genotype & phenotype had yet to be tightly linked, i.e. in which translation was inaccurate or not well-developed. The last common ancestor, and even the last common ancestors of each of the 3 Domains, may have been progenotes, or progenotes may have predated the LCA, as shown above.

But the belief that most of evolution (at least biochemical evolution) occurred before the last common ancestor creates a time problem. The Earth's formative meteor/comet bombardment ended ca. 3.8 Bya. Prokaryotic fossils from 3.6 Bya and cyanobacterial fossils (and oxygen isotope signals) from 3.2Bya have been found, so life apparently originated & most of evolution occurred in a fraction of the Earth's history. Some people interpret this as evidence that life originated elsewhere in the solar system (or beyond) and came to Earth on meteorites. I'll let you judge the idea of "Panspermia" for yourself. But at least from my perspective, there is probably no need for life to have originated elsewhere.One can imagine a quick, simple origon and rapid evolution and diversification. As organisms subsequently became increasingly complex, the rate of evolution would be expected to slow - complex systems evolve slower than simple systems. You see this all the time; for example, animal evolution was rapid at it's emergence at the Cambrian explosion, and hasn't changed much since. Or think about the automobile, or computers, or software. Any new "technology" evolves very quickly when it emerges and explores the available landscape, then slows down dramatically when it gets complex, and especially when the interactions it's invoved with get intricate.

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