"In biology, nothing makes sense without the theory of evolution." That's a close paraphrase of what Theodosius Dobzhansky wrote, and it's hard to disagree with him. Before Charles Darwin and Alfred Russel Wallace came up with the theory of evolution by natural selection, all the different species of plants and animals were simply present on earth. No one had any explanation of how they came to be, how they might change over time, or how they were related to each other.

With evolution in hand, though, you could build a nice, neat picture. Plants and animals slowly evolved from the simplest forms to the most complex. A "Tree of Life" showed development from single-celled animals like the amoeba, on to multi-celled animals, then to fishes with backbones, to amphibians, to reptiles, and finally to mammals and at last humans - we, naturally, being drawn at the very top of the tree.

That idea carried us from 1859, when "The Origin of Species by Natural Selection" was published, until 1953, when Francis Crick and James Watson proposed a structure for DNA and provided a molecular basis for the way that reproduction and evolution could proceed.

At first sight, it seemed that DNA, with its billions of four simple molecules repeated along a central spine, made things even more neat and tidy. The particular order of the molecules decided the nature of an organism, and the whole DNA was sufficient to define the organism exactly. Not only could we draw a Tree of Life, we could tell what lay on each branch, and how far apart the branches were.

Things didn't begin to get messy until the tools for genetic sequencing were developed. In the past twenty years those tools have allowed us to look, one by one, at every molecule in the DNA of a human or any other organism. What scientists find is bewildering.

First, there are chunks of DNA that seem to serve no purpose whatsoever. They are simply good at getting themselves copied, and they are repeated over and over in the whole DNA sequence. Second, there are sequences that look remarkably like the DNA of existing viruses. They seem to have somehow been trapped and preserved within the whole DNA structure, like a fly caught in amber. Theory followed that logic: the cells of an organism had found some way to neutralize a virus, originally invasive and perhaps harmful. The "fossil virus" now lives on within the parent organism, neither harmful nor useful.

Even with the explanation, the "Tree of Life" was beginning to look a bit messier, with bits that should be on one branch popping up on another, just as oak galls form part of an oak tree but house parasitic insects. And then, a few years ago, we caught a hint that things might be more complex yet. The information came from a particular form of parasitic wasp known as Cotesia congregata.

All parasitic wasps have certain habits that give most people the chills. In fact, the existence of these wasps supposedly was the reason that Charles Darwin became an atheist. The wasps sting caterpillars, grasshoppers, or worms, then lay their eggs inside them. Cotesia happens to lay its eggs inside a tobacco hornworm, easy to recognize when you see it on your tomatoes or other vegetables because it is fat and green and very big. The Cotesia wasp larvae use the flesh of the hornworm as food. However, the wasp sting does not kill the worm; it merely paralyzes it. The hornworm is then eaten alive, recovering from the paralysis enough to itself go on eating, even as it is internally consumed.

Very nasty. However, it raises an interesting question. The invaded hornworm has a good immune system of its own. Why doesn't that immune system attack the eggs and larvae of the wasp, recognizing them as foreign matter within its body?

It doesn't because when the wasp stabs the hornworm with its pointed egg-laying syringe, at the same time it injects the eggs it pumps in a large number of viruses. It is the viruses that take over some of the hornworm's functions, producing proteins that destroy the hornworm's own immune system and permitting the wasp's larvae to flourish unmolested.

And now for the strangest part of all. Where do the viruses come from that the wasp injects into the hornworm? You would expect them to be carried, intact, inside each wasp. They are not. Instead, bits of the genetic code of the virus are scattered throughout the wasp's own DNA. The whole virus is assembled within the wasp's ovaries, using the bits and pieces of genetic information drawn from those different sections. It invites a new question: Are the viruses independent organisms? Or are they merely fragments of wasp DNA, able to break free and wander away from the parent wasp, to fulfill a function vital to wasp reproduction within some other creature completely? If so, then they themselves are in some sense offspring of the wasp.

The more we learn of DNA sequences, the less our famous "Tree of Life" looks like a tree. The branches are braided and complex and intertwined, and every day we discover more examples of hopping from limb to limb. Bacteria constantly exchange DNA segments, in the form of ring structures known as plasmids. We and all animals contain within each of our cells energy-producing structures known as mitochondria, with their own DNA and their own reproductive cycles. Plants also have within them self- contained structures, the chloroplasts that contain chlorophyll and are responsible for photosynthesis. Perhaps oddest of all, Plasmodium, the minute animal that causes malaria, contains tucked away inside it something very like a chloroplast - a structure that we expect to find only in plants.

So the Tree of Life doesn't flourish in the same way as it did a century ago. What is replacing it is a far more dynamic and flexible view of Nature. Living things at all levels - including humans - depend on each other, invade each other, involve each other, and evolve in tandem. You and I think of ourselves as individuals, and we are; but we are also composite organisms.

Walt Whitman said, "I am large, I contain multitudes." I doubt if he knew how right he was.

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