Posted on : Aug-13-2014 | By : John | In : Book Club
This month we are reading Life Ascending: The Ten Great Inventions of Evolution by Nick Lane.
Nick Lane is a Reader in Evolutionary Biochemistry at University College London. He has written books about oxygen, mitochondria and cryobiology. Our current book is organized into 10 chapters, each covering an important advance (“inventions”) in the history of life. It is one of the most “sciency” books we’ve read recently, with occasional vivid descriptions of, for example, the view of the Earth from the Moon first seen by the Apollo 8 astronauts in 1968, the early Earth before the first life emerged and both kinds of hydrothermal vents (I didn’t know there was more than one.) There is little in the way of personal anecdotes or historical discussions. For the most part, the book dives right into the science, and there is a lot of it!
I’m finding it dense going but thoroughly worthwhile. A lot of it, especially the first chapter on the origins of life, is new to me. It also clears up a lot of common misconceptions, such as where the oxygen (O2) released by photosynthesis comes from. (It comes from splitting water molecules, not from reducing CO2. But it does convert atmospheric CO2 into solid carbohydrates, which is why plants are the ultimate solution to global warming.)
The first chapter concerns not an invention of evolution (unless non-living chemistry can be said to evolve), but the invention of life itself, abiogenesis. My knowledge must have been way out of date. I was familiar with the notion that life may have originated in undersea hydrothermal vents, possibly involving clay or another mineral substrate, utilizing organic molecules generated by mechanisms similar to the famous Miller-Urey experiment, but with loads of gaps and speculation. Much more is known now, and the speculations are far more detailed and have much more evidence to support them. For example, there are at least two kinds of hydrothermal vents. The more familiar “black smokers” occur above undersea volcanoes at hot spots (such as the Galapagos, Hawaii and Iceland) with giant worms and clams feeding on sulfur-reducing bacteria, but the more likely candidate sites are the cooler and less spectacular but more wide-spread white smokers, which emerge near sites of sea floor spreading at mid-ocean ridges. (The chemistry is different at the two types of vents.) A major problem with the older “primordial soup” hypothesis was thermodynamic. Without active chemistry and an energy source, the organic molecules produced in the Miller-Urey experiment (or by other mechanisms, such as accretion from comets) gradually degrades. It could never self-organize into the predecessors of life. The white smokers provide the energy and chemical activity to solve this problem. In addition, the minerals deposited there provide hollow “cells” where the necessary reaction products and accumulate at high concentrations, rather than dissipating into the ocean. Finally, the necessary precursor compounds to drive the Krebs cycle (the process that all living organisms use to generate ATP, the essential molecule for storing and transporting energy in virtually every biochemical reaction). Finally, the essential precursors to RNA, a nucleic acid polymer which, in the right environment, can act as a template for its own replication, were there. That’s all in the first chapter!
The second chapter is about the invention of DNA. This chapter is a little less speculative. DNA, a close relative of RNA, is useful for replicating cells. It is much more stable than RNA, meaning that it can be copied many times through many generations without becoming irrevocably corrupt. It originally seems to have been a sort of Time Machine backup for the RNA. In the RNA world, a “cell” could grow, building up many copies of its RNA and various metabolic products (such as ATP), and when it filled its space (a tiny hollow spot in the rocks), it could spill over into the next available, empty “cell”. But the error rate in copying the RNA is so large that it would basically be a genetic sludge. Any “useful” mutations would be totally swamped by the random changes. DNA provides a mechanism to more faithfully reproduce the RNA. The original cell makes DNA copies of its RNA, using an enzyme like reverse transcriptase (which is how modern retroviruses work), replicate the DNA, and then (when needed) make true copies of the original RNA by transcribing the DNA. All the work of the cell is done by the RNA (which in some sense is still true today), and DNA is just a backup copy on resilient media. Once DNA became the standard means of storing biological information, evolution had a chance to work.
The third chapter is about the evolution of photosynthesis. Originally, it was a process for using external energy to metabolize hydrogen and hydrogen sulfide and other commonly available compounds. Converting water and carbon dioxide into oxygen and carbohydrates using sunlight came later. In fact, water requires so much energy to break down that photosynthesis does it twice, by running through a two-stage process (capturing photons at each stage) before the “modern” (i.e. several billion year old) version of chlorophyll pops out a carbohydrate molecule. The original, one-stage version got replicated and ganged together to create the “Z scheme” (so named because of the up-and-down changes in electron energy.
All this happened when all life on Earth was single cell bacteria (or more likely their progenitor archaea.) The fourth chapter deals with the emergence of the complex (eukaryotic) cell (after about 3 billion years of boredom), with nuclei, mitochondria, chloroplasts, vesicles and all the other complex structures. This appears to have happened because a big bacteria ate a small bacteria, but got indigestion and failed to break it down and absorb it! Eukaryotes soon (in a mere 400 million years) banded together to form multicellular life and the Cambrian explosion.
The next chapters deal with such major innovations as Sex, Movement, Sight, Thermal control, Consciousness (Brains!) and finally, Death. This is a lot more ground to cover, I hope I finish in time! I think we will really enjoy discussing this book.
We’ll be meeting in ten days, but if you haven’t started it yet, it is still worth reading as much as you can. I hope everyone finds it as fascinating and informative as I have.
Pleas join us to discuss this book on Saturday, August 23 at 3 PM in our usual meeting place, Harvard’s Northwest Science Building, 52 Oxford Street, Cambridge. Bring your appetite and, if you wish, a snack to share. Also optional, you can RSVP on our Facebook event page. Remember, the first gratuitous Star Trek reference always receives a complimentary phaser blast. (Set to stun unless I remember to put new batteries in my phaser.)
Mary will be leading an online discussion of the book the next day (August 24) at the Skepchick Book Club. Drop by and make all those insightful comments you forgot to make at the meeting, or if you live too far away to attend in person. (But it’s much more fun to be there!)