My professional introduction to academia happened in the early 1980s, during my college years, when I volunteered at the American Museum of Natural History in New York City. Aside from the excitement of working behind the scenes in the collections of the museum, one of the most memorable experiences was attending their raucous weekly seminars. Each week a speaker would come to present some esoteric study on natural history. Following the presentation, often a fairly low-key affair, the listeners would pick the talk apart point by point. It was merciless. On occasion, the whole thing felt like a human barbecue, with the invited speaker as the spit-roasted main course. Frequently, these debates would devolve into shouting sessions with all the high dudgeon and operatic pantomime of an old silent movie, complete with shaken fists and stomped feet.
Here I was, in the hallowed halls of academe, listening to seminars on taxonomy. You know, taxonomy—the science of naming species and organizing them into the classification scheme that we all memorized in introductory biology. I could not imagine a topic less relevant to everyday life, let alone one less likely to lead eminent senior scientists into apoplexy and the loss of much of their human dignity. The injunction "Get a life" could not have seemed more apt.
The irony is that I now see why they got so worked up. I didn't appreciate it at the time, but they were debating one of the most important concepts in all of biology. It may not seem earth-shattering, but this concept lies at the root of how we compare different creatures—a human with a fish, or a fish with a worm, or anything with anything else. It has led us to develop techniques that allow us to trace our family lineages, identify criminals by means of DNA evidence, understand how the AIDS virus became dangerous, and even track the spread of flu viruses throughout the world. The concept I'm about to discuss supplies the underpinning for much of the logic of this book. Once we grasp it, we see the meaning of the fish, worms, and bacteria that lie inside of us.
The articulation of truly great ideas, of the laws of nature, begins with simple premises that all of us see every day. From simple beginnings, ideas like these extend to explain the really big stuff, like the movement of the stars or the workings of time. In that spirit, I can share with you one true law that all of us can agree upon. This law is so profound that most of us take it completely for granted. Yet it is the starting point for almost everything we do in paleontology, developmental biology, and genetics.
This biological "law of everything" is that every living thing on the planet had parents.
Every person you've ever known has biological parents, as does every bird, salamander, or shark you have ever seen. Technology may change this, thanks to cloning or some yet-to-be-invented method, but so far the law holds. To put it in a more precise form: every living thing sprang from some parental genetic information. This formulation defines parenthood in a way that gets to the actual biological mechanism of heredity and allows us to apply it to creatures like bacteria that do not reproduce the way we do.
The extension of this law is where its power comes in. Here it is, in all its beauty: all of us are modified descendants of our parents or parental genetic information. I'm descended from my mother and father, but I'm not identical to them. My parents are modified descendants of their parents. And so on. This pattern of descent with modification defines our family lineage. It does this so well that we can reconstruct your family lineage just by taking blood samples of individuals.
Imagine that you are standing in a room full of people whom you have never seen before. You are given a simple task: find out how closely related each person in the room is to you. How do you tell who are your distant cousins, your super-distant cousins, your great-granduncles seventy-five times removed?
To answer this question, we need a biological mechanism to guide our thinking and give us a way to test the accuracy of our hypothesized family tree. This mechanism comes from thinking about our law of biology. Knowing how descent with modification works is key to unlocking biological history, because descent with modification can leave a signature, which we can detect.
Let's take a hypothetical humorless, quite unclown-like couple who have children. One of their sons was born with a genetic mutation that gave him a red rubber nose that squeaks. This son grows up and marries a lucky woman. He passes his mutated nose gene to his children, and they all have his red rubber nose that squeaks. Now, suppose one of his offspring gets a mutation that causes him to have huge floppy feet. When this mutation passes to the next generation, all of his children are like him: they have a red rubber nose that squeaks and huge floppy feet. Go one generation further. Imagine that one of these kids, the original couple's great-grandchild, has another mutation: orange curly hair. When this mutation passes to the next generation, all of his children will have orange curly hair, a rubber nose that squeaks, and giant floppy feet. When you ask "Who is this bozo?" you'll be inquiring about each of our poor couple's gre at- gre at- grandchildren.
This example illustrates a very serious point. Descent with modification can build a family tree, or lineage, that we can identify by characters. It has a signature that we immediately recognize. Like a nested set of Russian dolls, our hypothetical lineage formed groups within groups, which we recognize by their unique features. The group of "full bozo" great-great-grandchildren is descended from an individual who had only the squeaky nose and the huge floppy feet. This individual was in a group of "proto-bozos," who are descended from an individual who had only the rubber nose that squeaks. This "pre-proto-bozo" was descended from the original couple, who didn't look overtly clown-like.
The bozo family tree.
This pattern of descent with modification means that you could easily have hypothesized the bozo family tree without me telling you anything about it. If you had a room full of the various generations of bozos, you would have seen that all clown kin are in a group that possesses a squeaky nose. A subset of these have orange hair and floppy feet. Nested within this subset is another group, the full bozos. The key is that the features—orange hair, squeaky nose, big floppy feet—enable you to recognize the groups. These features are your evidence for the different groups, or in this case generations, of clowns.
Replace this family circus with real features—genetic mutations and the body changes that they encode—and you have a lineage that can be identified by biological features. If descent with modification works this way, then our family trees have a signature in their basic structure. So powerful is this truth that it can help us reconstruct family trees from genetic data alone, as we see from the number of genealogical projects currently under way. Obviously, the real world is more complex than our simple hypothetical example. Reconstructing family trees can be difficult if traits arise many different times in a family, if the relationship between a trait and the genes that cause it is not direct, or if traits do not have a genetic basis and arise as the result of changes in diet or other environmental conditions. The good news is that the pattern of descent with modification can often be identified in the face of these complications, almost like filtering out noise from a radio signal.
But where do our lineages stop? Did the bozos stop at the humorless couple? Does my lineage stop at the first Shubins? That seems awfully arbitrary. Does it stop at Ukranian Jews, or northern Italians? How about at the first humans? Or does it continue to 3.8-billion-year-old pond scum, and beyond? Everybody agrees that their own lineage goes back to some point in time, but just how far back is the issue.
If our lineage goes all the way back to pond scum, and does so while following our law of biology, then we should be able to marshal evidence and make specific predictions. Rather than being a random assortment of creatures, all life on earth should show the same signature of descent with modification that we saw among the bozos. In fact, the structure of the entire geological record shouldn't be random, either. Recent additions should appear in relatively young rock layers. Just as I am a more recent arrival than my grandfather in my family tree, so the structure of the family tree of life should also have its parallels in time.
To see how biologists actually reconstruct our relatedness to other creatures, we need to leave the circus and return to the zoo we visited in the first chapter of the book.
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Although nobody gets a parenting manual or bible in the delivery room, it is our duty as parents to try to make our kids as well rounded, happy and confident as possible. It is a lot easier to bring up great kids than it is to try and fix problems caused by bad parenting, when our kids have become adults. Our children are all individuals - they are not our property but people in their own right.