The Vital Question is a lively presentation, but it fits in a substantial amount of material and assumes at least a general familiarity with chemistry and biology, with basic cell biology, genetics, and suchlike. (There is a glossary, but that's more useful as a reminder, for those already familiar with the terms, than as a primer.) It is a nice summary of current research, with no attempt to be comprehensive but a balance between ideas that are now widely accepted ideas and some more speculative ones.
Two chapters form a kind of introduction. "What is Life?" offers a basic history of life on Earth, along with an introduction to phylogenetics. Central here are the ideas of Lynn Margulis and Carl Woese, the division of life into three kingdoms, with the third originating in a symbiosis between members of the first two, between an archaeal host cell and a bacterial endosymbiont. "What is Living?" turns to requirements for living, particularly energy. Living organisms all get energy from proton gradients and the most conserved proteins are not involved with DNA replication, but are components of the energy system: notably proton pumps and ATP-synthase.
Following up this focus on energetics, Lane presents a compelling case for the origin of life in hydrothermal vents, as natural flow reactors.
"natural proton gradients drove the origin of life on earth in a very particular environment, but an environment that is almost certainly ubiquitous across the cosmos: the shopping list is just rock, water and CO₂".
He then turns to the emergence of cells, where the explanation for the radically different membranes of bacteria and archaea is that membranes came late:
"LUCA (the Last Universal Common Ancestor) really was chemiosmotic, with an ATP synthase, but really did not have a modern membrane, or any of the large respiratory complexes that modern cells use to pump protons. She really did have DNA, and the universal genetic code, transcription, translation and ribosomes, but really had not evolved a modern method of DNA replication."
He explains the importance of leaky membranes, allowing flows to keep going, and lays out a plausible account of the development first of an antiporter, swapping sodium and hydrogen ions, and then of a proton pump, creating an environment which would favour modern cell membranes.
Moving on to the much later origin of complex cells, Lane again sees energetic constraints as central.
"The origin of the eukaryotic cell was a singular event. ... When considered in terms of genomes and information, this particular trajectory is nearly impossible to understand. But when considered in terms of energy and the physical structure of cells it makes a great deal of sense."
Eukaryotes have as much as 100,000 times as much energy per gene as prokaryotes, which is a result not just of an increased membrane surface area, but also of the efficiencies arising from mitochondrial gene loss. (Lane argues that mitochondria have retained some genes because of the need to control the local chemiosmotic coupling and membrane potential.)
In "Sex and the Origins of Death" Lane argues that much of the complexity of eukaryotic genomes results from their mixed inheritance. Introns are the result of "a barrage of genetic parasites" that early eukaryotes faced from their own endosymbionts; nuclei evolved as a defence against this, allowing spliceosomes to remove introns from transcribed messenger RNA before ribosomes can translate them into proteins. Turning to sex, he argues that many of its peculiar features can be explained if "the ancestral state was biparental, anisogamy arose next, then uniparental inheritance, and finally the germline".
"Mitochondrial variation alone can explain the evolution of multicellular organisms that have anisogamy (sperm and eggs), uniparental inheritance, and a germline, in which female germ cells are sequestered early in development — which together form the basis for all sexual differences between males and females."
A final chapter "The Power and the Glory" considers some of the implications of mitochondria and the respiratory chain being "mosaics": apoptosis, hybrid breakdown and speciation, sex determination and Haldane's rule (that when one sex is missing it is the heterozygous one), and ageing and variation in lifespans. And an epilogue offers some speculation about a recent, one-off discovery of a hard-to-classify single-celled Parakaryon myojinensis
The Vital Question is a spellbinding tour of some of the most exciting research being done in biology, a lively presentation which had quite a bit that was new to me in it. It will be compelling reading for anyone curious about the origins and foundational history of life.
March 2017
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