Evolution

The "whale" was not a true whale, the "dolphins" not dolphins. Those great marine mammals had preceded humanity into extinction. These creatures were descended from birds: In fact, from the cormorants of the Galapagos Islands in the Pacific, which, blown there from mainland South America by contrary winds, had given up flight and taken to exploiting the sea. Their descendants’ wings had become fins, their feet flukes, their beaks a variety of specialized instruments — snappers, strainers — for extracting food from the ocean. Some of the species of "dolphin" had even regrown the teeth of their ancient reptilian ancestors: The genetic design for teeth had lain dormant in birds’ genomes for two hundred million years, waiting to be re-expressed when required.

Invisibly slow on any human timescale, adaptation and selection were nevertheless capable, given thirty million years, of turning a cormorant into a whale, a dolphin, or a seal.

And, strangely enough, all the swimming birds Remembrance saw were indirect legacies of Joan Useb.

As Remembrance watched, a dolphinlike creature erupted out of the water right in the middle of the cloud of flying fish. The fish scattered, their fin-wings buzzing, but the beak of the "dolphin" snapped closed on one, two, three of them before its sleek body fell back into the water.

The sun was starting its long descent toward the sea. Remembrance stood up, brushed herself free of sand, and resumed her cautious knuckle-walk along the fringe of the beach, but something overhead distracted her. She glanced up at the sky, fearing it was another bird of prey. It was a light like a star, but the sky was still too bright for stars. As she watched, it slid over the roof of the sky.

The light in the sky was Eros.

NEAR, the humble, long-dead probe, had spent thirty million years swimming with its asteroid host through the spaces beyond Mars. Its exposed parts were heavily eroded, metal walls reduced to paper thinness by endless microscopic impacts. At the touch of a gloved astronaut’s hand it would have crumbled like a sculpture of dust.

But NEAR had survived this far, among the last of all of mankind’s artifacts. If Eros had kept up its eccentric dance around the sun, perhaps NEAR could have survived longer yet. But it was not going to get that chance.

The asteroid’s passage through the atmosphere would be mercifully swift. The fragile probe, returning to the planet where it had been made, would flash to vapor only fractions of a second before the great body with which it had long ago rendezvoused was itself destroyed.

Earth’s evolutionary laboratories had been stirred many times by monstrous interventions from without. Now, here was another stirring. And over the bright scene on which Remembrance gazed, a curtain would soon be drawn.

Remembrance herself would survive, as would the children she would bear in the future. Once again the great work would begin: Once again the processes of variation and selection would sculpt the descendants of the survivors to fill shattered ecological systems.

But life was not infinitely adaptable.

On Remembrance’s Earth, among the new species there were many novelties. And yet they were all variations on ancient themes. All the new animals were built on the ancient tetrapod body plan, inherited from the first wheezing fish to have crawled out of the mud. And as creatures with backbones, they were all part of a single phylum — a great empire of life.

The first great triumph of multicellular life had been the so-called Cambrian explosion, some five hundred million years before the time of mankind. In a burst of genetic innovation, as many as a hundred phyla had been created: each phylum a significant group of species representing a major design of body plans. All backboned creatures were part of the phylum of chordates. The arthropods, the most populous of the phyla, included creatures like insects, centipedes, millipedes, spiders, and crabs. And so on. Thirty phyla had survived life’s first great shaking down.

Since then species had risen and fallen, and life had suffered major disasters and recoveries over and over again. But not one new phylum had emerged, not one, not even after the Pangaean extinction event, the greatest emptying of all. Even by the time of that ancient event, life’s capability for innovation was much constrained.

The stuff of life was plastic, the mindless processes of variation and selection inventive. But not infinitely so. And with time, less.