Life Ascending

How did we come to be here, conscious animals on a planet bursting with life? There's a long story and a short one. The long story is our planet's history, a 4.5 billion year epic of unimaginable complexity that defies being told in a single book. Then there's the short story, the story of the few seminal "inventions" of evolution from which everything else flowed. I outline this story in my new book, Life Ascending: The Ten Great Inventions of Evolution.

Of course anyone can choose their own list of life's great inventions, and my list is personal, however well I may justify it. Nonetheless, each of these inventions transfigured the world, ultimately making our own existence possible. Here's the short story.

1. The Origin of Life

A 30-metre tall active alkaline vent. Reproduced with permission from Deborah S. Kelley and the Oceanography Society

The origin of life is one of biology's biggest conundrums. How prebiotic chemistry gave rise to biochemistry, how the first cells formed, what kind of energy first powered metabolism and replication -- all these questions are serious challenges. Remarkably, all are answered in broad brush stroke by the amazing properties of alkaline hydrothermal vents, which form naturally chemiosmotic, self-replicating mineral cells with catalytic walls. They concentrate organics, including nucleotides, in impressive quantities, making them the ideal hatcheries for life.

2. DNA

DNA is unique. RNA, chemically very similar, is far more unstable and reactive and couldn't encode organisms much more complex than a virus. For life to get going, DNA was needed. How a primordial RNA world gave rise to DNA and proteins is one of the great questions in biology. Yet a "code within the codons" gives suggestive clues to the origin of DNA and also points to life's origin in alkaline hydrothermal vents. A deep distinction in DNA replication mechanisms and other traits imply that bacteria and archaea emerged independently from a common ancestor in the vents.

3. Photosynthesis

Without photosynthesis life couldn't get very far. Photosynthesis provides both the fuel and oxygen for respiration -- and only aerobic respiration generates enough energy to support multicellular life. Oxygenic photosynthesis arose just once in the history of evolution, in cyanobacteria. The trick demands an elaborate biochemical scheme to extract electrons from water and thrust them onto carbon dioxide. Without that improbable pathway, we would not be here.

4. The Complex Cell

Living stromatolites, constructed by cyanobacteria. Image: Courtesy of Catherine Colas des Francs-Small University, Western Australia.

All complex life on Earth is composed of nucleated cells, known as eukaryotic cells. The eukaryote arose only once, and bacteria normally show no tendency towards morphological complexity. The last common ancestor of eukaryotic cells was a chimera, formed in a unique union between two prokaryotic cells called endosymbiosis -- a non-Darwinian mechanism whereby organisms converge rather than diverge. Without that chimera, evolution may never have progressed beyond bacteria, and again none of us would be here.

5. Sex

Sex is absurd. It costs a small fortune to find a partner, transmits foul venereal diseases and parasitic genes, and randomises successful allele combinations. Worse, sex requires males, viewed by implacable feminists and evolutionists alike as a waste of space. Why we all have sex anyway was seen as the queen of evolutionary problems in the 20th century. Recent work shows that over time all complex species would degenerate like the Y chromosome without sex. The details help explain why sex first arose, enabling early eukaryotes to thrive.

6. Movement

Muscles set animals apart. They power grazing and predation and make food webs a reality. The proteins responsible for contractility -- actin and myosin -- are ubiquitous in eukaryotes and even in bacteria, propelling amoebae around, supporting plant cells, and helping bacteria divide. Actin forms dynamic cross links in much the same way that variant haemoglobin distorts red cells in sickle-cell anaemia. Selection fashioned such spontaneous quirks into the might of muscle.

7. Sight

A fruit fly's eye Image: Courtesy of Walter Gehring, Biozentrum, University of Basel, Switzerland.

Sight may well have been the driving force behind the Cambrian explosion, when the first animals leapt into the fossil record about 550 million years ago. Thanks to a series of surprises in molecular biology, we now know how eyes evolved in great detail. Lens proteins and crystals were recruited from an astonishing range of sources, from calcite to mitochondria to stress proteins, but the ubiquitous light-sensitive protein rhodopsin probably evolved in algae, where it is used to calibrate light levels in photosynthesis.

8. Hot Blood

Endothermy drives a supercharged lifestyle, making our own 24/7 dynamism possible. Many small mammals eat as much in a day as a lizard does in a month. A big benefit is stamina, but there is no necessary connection between stamina and resting metabolic rate, and theropods like Velociraptor may have had the best of both worlds. One driver for endothermy may have been diets rich in carbon but low in nitrogen, such as leaves. Herbivores gain enough nitrogen from leaves if they eat a lot and jettison the excess carbon. Endotherms cleverly burn it off, gaining stamina while subsisting on a lower quality diet.

9. Consciousness

Consciousness is the most subversive evolutionary adaptation. It enabled us to transform the world -- but there are still deep uncertainties about what it actually is. We don't know yet how neurons firing in the brain can generate a feeling of anything: what, if anything, a feeling is in physical terms. This is what philosophers call the "hard problem," and some say answering it requires a radical overhaul of the laws of physics. The answer may lie in bees, which have complex neural reward systems -- they may not be truly conscious, but if they feel anything at all, they already possess the physiological rudiments of consciousness.

10. Death

Without death, natural selection would count for nothing, and life could never have evolved at all. Without cell death, or apoptosis, multicellular organisms are not possible. The key to both is mitochondria. They generate reactive free radicals that slowly undermine health, but in the short term optimise respiration, enhancing fitness when young. The penalty for vigour in youth is decrepit old age. There's hope. Birds leak fewer free-radicals, and live longer than mammals, without losing their vigour. The anti-aging pill may not a myth.



Are these the best ten evolutionary inventions? You might disagree, but each one on my list transformed our planet, overwriting previous revolutions with new layers of complexity. Each dominates our lives today, each is scientifically and culturally iconic, and each evolved by natural selection. While fascinating in their own right, together they tell the remarkable story of life on Earth. More dramatic, more compelling, more intricate than any creation myth, this story has the added advantage of being, to a first approximation, true.



Life Ascending: The Ten Great Inventions of Evolution, by Nick Lane, Profile Books, London, 2009. 288 pp. ISBN: 978-1-861-97848-6. £18.99.

Nick Lane is a biochemist and honorary reader at University College London. His previous books include Oxygen and Power, Sex, Suicide, and he's been described by Nobel Laureate Frank Wilczek as "a writer who's not afraid to think big -- and think hard." Lane's current research is on the constraints imposed by chemiosmosis in the origin and evolution of the eukaryotic cell.

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