Creation (6 page)

The fact that a mirror world could exist but doesn't shows forcefully that life is of singular origin. If there ever was a choice between left and right, only one was selected, and the other discarded, never to feature again in the natural course of things. At the beginning of these biological mechanics, it might be as simple as a flipped coin, a chance event that was followed with absolute fidelity forever.

The Singularity

The origin of cells from parent cells and the origin of species via slowly changing genes in those cells both bear the hallmarks of a single origin. Those three aspects of biology—cells only from existing cells, DNA changing through imperfect copying, and modified descent of a species as a result—logically unveils a single line of ancestry that inevitably leads back to a single point in our deep, deep past.

In other words, any one of the new cells that were pulled from their parent as a result of the orchestrated disaster response to a paper cut bears a direct and noble lineage. Life is replete with extinction, but a new cell on your hand is one of the great survivors of the longest pedigree in history. In one generation its ancestry traces back to the fertilized egg from which every cell in your body was born. The egg and sperm that fused to form your unique combination of genes and DNA can trace a similar pathway back to their fertilized egg origin, and so on through their parents, grandparents, and every ancestor in your family tree—and, indeed, the history of our species.

Naturally, it doesn't nearly end there. These cells, in turn, trace their ancestry through the life span of our apelike ancestors. We don't know exactly who they were yet, but they existed in the bodies of individuals who first carved bones to make tools, started fires with flint, and stood upright as we, among apes, uniquely do.
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And it certainly doesn't end there, either. Before being in the loins of that upright ape, your cell lineage was carried through long-gone primates, maybe something like
Proconsul
, which resembled something between a chimpanzee and a macaque. Before them, it lay in monkeys and earlier through their furry, wet-nosed ancestors, more like modern lemurs. And as we follow them back, at some point, cells that were once the predecessors of your Pac-Man macrophages or sparking neurons existed in a shrewlike creature with fur and nipples. That critter, your ancestor, was present when an enormous meteor thundered into the Tropic of Cancer and called time on the reign of the dinosaurs sixty-five million years ago. Your cell lineage witnessed the entire fall and rise of those beasts inside many forms of early mammals, such as the
Cynodontia
, which looked like a snarling rodent of unusual size. Before those first mammals appeared more than 220 million years ago, your cell's ancestor was a shelled egg of a reptile, hairless and cold-blooded, such as
Diadectes
, a six-and-a-half-foot-long brute that looked like a stocky, diet-shy crocodile.

To get to be in that egg, your cell survived iterations of beasts that had, among many other things, evolved cells that produce a thicker collagen framework in their breathing organs; with this innovation the primitive lungs could support themselves without the need for water. Your cell line may well have passed through a creature like the 375-million-year-old lungfish
Rhinodipterus
, which, unlike other fish at that time, had a muscular neck, which enabled it to raise its head above the water line. These innovations allowed it to breathe air rather than suck oxygen from the seas, and maybe opened up a whole new world of food. Further back your cell line was contained within a much more fishlike thing, with fins and gills. Before that it was in one of the first vertebrates, a swimming thing that looked a bit like an eel or a lamprey. And before that it was in a much wormier thing, similar to the living two-inch amphioxus: in one of those cells a massive copying error in its genome, a quadrupling of its entire DNA, resulted not in a fatality, but in a whole genetic platform on which vertebrates began to evolve. Before that it was in a spongy thing. And before that, just a clump of cells free-floating or lodged on a rock. And back and back and back ceaselessly into the past. The lineage of your cells has survived every disaster, catastrophe, meteorite, every extinction, ice age, and hungry predator—every event in this solar system for almost four billion years.

The vast majority of your cells, including all of the new ones in your cut, are terminal branches in this absolute pedigree, as their story will end with your life. The only ones that survive to give rise to cells in the next generation are sperm or egg. And of the trillions of cells that have worked for you throughout your life, only a handful will go on, the lucky few that will meet a sperm or egg and make a child. But the information within all of them will be carried on. Down that line has been transmitted the DNA that builds all of those cells together to make the most efficient way of ensuring the perpetual existence of your genes: an organism. Life is an astonishingly conservative system: DNA is the same in all species; the letters of the code are all the same; the encryption in the code is the same; even the orientation of the molecules is the same. What's true in bacteria is true in a blue whale. Only a system with a single root could display such conservation.

That pathway backward, retracing our steps as they become fainter and fainter over geologic time, can be applied to any creature alive today, or ever. The gaps become bigger, and it's almost always only hypothetical. Although we have a good overall understanding of the origin of species, to claim one species was the direct ancestor of another is often overstating what we can know. But the broad sweep of evolution is well understood, and any step backward through the past from any creature drives us to a single conceptual home. The branching tree of life ultimately becomes narrower as we reverse through time until we reach a single root.

We could trace an equivalent route backward for a cell taken from the boiling mud in an Icelandic hot spring, from the flower of a sweet pea, or from a button mushroom from the supermarket, and every time we would end up back in the same place. In every cell is a perfect unbroken chain that stretches inevitably back to the origin of life. That lineage irresistibly leads back to one single entity, which we call the Last Universal Common Ancestor, or Luca. Somewhere on the infant earth, Luca split in two. Since then, the thing that we struggle to define as life has passed uninterrupted from it to you, via a colossal series of iterations. Existence is bewilderingly tenacious.

Entities that we might be able to describe as living may have emerged several times, but life only enduringly survived once, and then continuously. We are confident of this because these other forms simply do not exist. At least they have not been discovered: there is a branch of speculation that proposes the rather futuristic-named shadow biosphere. This is the idea that there is a second (or more) undetected tree of life on Earth, with hallmarks different from the ones on the only tree of life we know of. But as it is, every life-form so far examined is based on cells, DNA, and Darwin. Discovery of a second tree of life here on Earth would give much-needed credence to the search for life on other planets, as it would double the number of known successful origin of life events. It would show that we are not a fluke. However, science is based on observable evidence. Therefore, the shadow biosphere, while sounding quite thrilling, is resolutely science fiction.

Pruning the Tree of Life

The next questions seem glaringly obvious. What was Luca, and where did it come from? We can reasonably assume that it had DNA as its genetic code, as all creatures after Luca do, and they are unlikely to have evolved that mechanism independently.

These ideas are largely based on what we share with other lives, whether it is the orientation of molecules, big physical characteristics such as five-digit limbs, or even simpler things such as having a head at one end and a tail at the other. Since the era of DNA began, and especially once the technology for reading genomes became much more accessible in the 1990s, studying evolution has been fortified by comparing similarities and differences in the precise letters of DNA. Because we have shared ancestry, thousands of our genes are very similar in closely, or even distantly, related species.
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DNA acquires copying errors at a fairly steady rate, which means that we can compare DNA in any living species and figure out when they separated. We can compare genes and protein sequences from any two organisms and calculate how long they have been diverging. In this way, we can reconstruct history in exactly the same way as paleontologists do with fossil bones, by looking at similarities and differences, and assembling all those comparisons to show not just relatedness between two species, but when the schism occurred. This is called phylogenetics and has utterly confirmed Darwin's ideas about the branching tree of life.

However, biology is littered with exceptions of varying sizes, and when it comes to the tree of life, there is one colossal exception. Using phylogenetics, many scientists now argue persuasively that for the first billion years or so, life was not so much of a branching tree but a tangled bush.

The first life-forms, for the first couple of billion years following their split from Luca, were single cells. They were evolving, but not really transforming radically. In fact, despite reproducing thousands of times faster than most animals, for the first couple of billion years, life failed to make it past the stage of microbes. These two domains of life are archaea and bacteria—superficially similar things, both single-celled entities of roughly the same size. For a long time, archaea were similar enough to bacteria not to be recognized at all. But they are different enough to now be classed as very distinct from bacteria and, indeed, from everything else (and we will find that these differences are crucial in the theory of the origin of life). As a domain, they are a separate category at the highest rank of how we categorize life. The great leap forward occurred with the arrival of complex life. This branch of the tree, the third domain, is called eukaryotes, and includes everything that isn't in the first two, including you and me and yeast and snakes and algae and fungi, flowers, trees, and turnips. At some point, maybe some two billion years ago, complex life emerged when an extremely unlikely pairing occurred: an archaea swallowed a bacteria. Rather than death of one or both, mutual benefit was the result. The consumed ceased being a free-living entity and was permanently annexed into the guts of what would grow to become the third domain of life, the one that you are in.

This idea was first presented by the American biologist Lynn Margulis in 1966. It caused rousing controversy and was largely rejected, and she was considered to be heretical. With time, experiments, and evidence, her views were vindicated and are now the orthodoxy. The evidence comes in a number of forms, most simply that complex cells, from which we and all animals are created, contain small internal power units called mitochondria. The processes that happen inside these power stations are of fundamental importance to the origin of life, but we will come to that in due course. Mitochondria function as chemical engines that provide energy for the cell and, by extension, the organism. In simple terms, mitochondria resemble bacteria. They are roughly the same size as bacteria and, just like bacteria, have circular strings of DNA as their own genome, which are independent of the vast majority of a cell's genetic information, safely housed in the nucleus.

That engulfing of one life-form into the other was not a meal, but a hostile takeover. The engulfed would never be free again, but enabled previously impossible growth in its new host. It came with its own genome, with thousands of genes. Over time, most have been lost to natural selection, or migrated to the host's nucleus control center. But the mitochondria retains an independent set of genes to this day, almost all of which are devoted to maintaining the energy generation for cells. When this happened, charged with new energetic vigor, genomes could grow and form greater templates for evolution beyond single cells. Cells could evolve internal structures and compartments that increased specialization of cells. From there, the acquisition of coordinated communication between cells meant that an organism was not restricted to a single cell. Multicellularity followed, eventually enabling the evolution body plans for plants and animals, complex networks of cross-talking cells that interact with one another and the environment in harmony.

In backtracking through the tree of life in order to work our way back to Luca, these events provide a problem. The complication in estimating the timing and a description at the base of life comes from the fact that both bacteria and archaea do something else scientists did not expect. We pass genes down only from cell to daughter cell, from parent to child. Bacteria and archaea can swap genes, and therefore characteristics, with other individuals. Sometimes they don't even need to be of the same species to do this. This is called horizontal gene transfer (as opposed to vertical descent), and it is critical in confounding our attempts to understand the origin of life. The reason for this is because these cells don't always gain evolved functions by the typical process of descent via cell division.
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The evolution of language is a handy metaphor here. The words
bigamy, bicycle,
and
biscuit
have a common root.
Bis
in Latin means “twice,” so you get twice married, ride on two wheels, and eat twice-cooked tasty snacks. But there are some words or phrases that are better expressed in other languages or even absent in the recipient, and so just get pinched.
Cul-de-sac
has a slightly different and more specific meaning in English than “dead end,” so we have adopted it. The German word
Schadenfreude
has no English counterpart, but is now a splendid word in itself for ignobly enjoying your enemy's misfortune. Yet in Swedish, the word is
skadeglädje,
derived and adapted directly from
Schadenfreude
but bypassing any common root. It has passed horizontally and subsequently evolved independently.