They can escape this dead end with little bursts of evolutionary renewal. Some of them steal genes from their host that allow them to build protein shells. They become viruses that can break free of their own cell and infect other ones. Some of these breakaways can even infect new species. They probably get carried away by parasites (such as mites) that take them to their new host, although some of the jumps are so long that it’s hard to know how they could possibly happen. How is it, for instance, that a freshwater flatworm has the same genetic parasites as a hydra living in the ocean, and a beetle living on land?
Viruses and genetic parasites may be common today, but 4 billion years ago parasitism might have been even more rampant. A typical organism alive today, be it a bacterium or a redwood, carries genes that are organized into powerful coalitions. They can copy themselves accurately into a new generation, and they can put up a fight against cheating genes. But when the Earth was young, some biologists think that genes were barely organized and couldn’t cooperate very well. Genes moved fluidly from one microbe to the next, sliding in and out of genomes through a sort of global microbial network. Any genes that could trick others into replicating them would be rewarded by natural selection and spread. Eventually the coalitions of genes got organized into separate organisms, but they were still trading DNA around so promiscuously that a biologist would have a hard time classifying them into separate species.
In spite of the assaults, true organisms did manage to evolve. Probably their genes evolved to a point where they all worked together well and could shut out cheating genes, and they could faithfully replicate themselves. It was probably at this time that life began to diverge into three great branches: bacteria, Archaea, and eukaryotes. Some of those early microbes found their energy in the chemicals growing along hydrothermal vents. As hundreds of millions of years drifted by, some lineages of bacteria became able to capture the energy of light. Other bacteria scavenged their microbial dung. Others evolved into killers, swallowing up the self-sufficient bacteria. Genetic parasites still lived off these different kinds of microbes, although their hosts had begun to get the upper hand.
But with every level of complexity that life achieved, a new kind of parasite emerged. When true organisms evolved, some of them became parasites. There are a few plausible stories of how they first evolved, and they may all turn out to be true in one case or another. One story begins with microbial predators swallowing what should have been their next meal. They opened up a cavity in their membrane and engulfed their prey; they prepared to carve it up, but for some reason, that was as far as their meals got. The prey sat in the predator’s microbial belly, indigestible.
Now the tables were turned—the prey turned out to be able to get a little nutrition from its failed predator before it was spat out. That extra food, that brief shelter from more successful predators, helped the prey reproduce more quickly than it would have otherwise. Natural selection would make the genes that helped it survive inside the predator became more common. They were joined by other genes that helped the prey actually seek out its predator, to open those cavities in the predator’s membrane by themselves. The prey spent more and more time inside the predator and gradually abandoned its free-living ways. Now it became the predators that had to fight off the prey, putting more and more effort into expelling them. If the cost of trying to fight off the invasion of parasites became too great, it would have benefited some hosts to make their parasites full-time guests. When the host divided, the parasite copied its own DNA and passed it down through the generations.
Once brought together this way, parasite and host can take their relationship in any one of several directions. The parasite may go on making its host’s life miserable, or it may instead become useful to the host, perhaps secreting some protein that the host can use. After many generations together, the lines between parasite and host may begin to blur. Some of the DNA of the parasite is accidentally ferried into the host’s own genes. The parasite itself may shrivel away to a few essential functions. The two organisms become essentially one.
Darwin never imagined this sort of fusion of life. He thought of life as an ever-branching tree, something like the tree shown on page 124. But biologists now recognize that they need to braid some of the branches together.
Scientists are now sequencing the full battery of genes in many microbes, and in them they can see signs of the choices that parasites have taken. Among the fully sequenced species is Rickettsia prowazekii, a bacterium that causes typhus. It invades cells, soaks up their nutrients and consumes their oxygen, multiplies like mad, and bursts its hosts open. Its DNA looks remarkably like the DNA in mitochondria, the organelles that provide every cell in our body with energy. A primordial free-living bacteria must have been the ancestor of both Rickettsia and mitochondria perhaps 3 billion years ago. Some of its descendants ended up passing through the earliest eukaryotes. The branch that led to Rickettsia evolved down the vicious path, while mitochondria’s ancestors eventually settled peacefully inside their hosts. Mitochondria was a fortunate parasite for our ancestors to gain. Photosynthesizing bacteria were gradually filling the atmosphere with oxygen, and mitochondria let eukaryotes breathe it.
Today’s eukaryotes are the product of a slow orgy of feasting and infection. After mitochondria invaded, several branches of eukaryotes all gained more bacteria of their own. These bacteria were photosynthetic, and their hosts stripped them down to their bare sun-harnessing essence, the chloroplast. These eukaryotes gave rise to algae and land plants, which added even more oxygen to the air. We can breathe oxygen, and plants can produce it in vast quantities, thanks to the parasites inside our cells.
This billion-year-old drama explains how malaria came to be a green disease. Some ancient eukaryote swallowed a photosynthesizing bacteria and became a sunlight-gathering alga. Millions of years later one of these algae was devoured by a second eukaryote. This new host gutted the alga, casting away its nucleus and its mitochondria, keeping only the chloroplast. That thief of a thief was the ancestor of Plasmodium and Toxoplasma. And this Russian-doll sequence of events explains why you can cure malaria with an antibiotic that kills bacteria: because Plasmodium has a former bacterium inside it doing some vital business.
It’s hard to know what exactly that ancient parasite did with its newfound chloroplasts. Perhaps it used them to live like a plant by photosynthesis. But that’s not the only possibility, because chloroplasts in plants do more than harness sunlight. They make many compounds, including fatty acids (the sort of molecules that constitute olive oil, for example). David Roos and his colleagues have speculated that in Plasmodium and Toxoplasma, their remnant of a chloroplast still makes these fatty acids and that the parasites use them to enshroud themselves inside their host cells. Clindamycin may be lethal to the parasite because it destroys Plasmodium’s bubble.