How can a walk through the zoo help us predict where we should look in the rocks to find important fossils? A zoo offers a great variety of creatures that are all distinct in many ways. But let's not focus on what makes them distinct; to pull off our prediction, we need to focus on what different creatures share. We can then use the features common to all species to identify groups of creatures with similar traits.
It took us six years to find it, but this fossil confirmed a prediction of paleontology: not only was the new fish an intermediate between two different kinds of animal, but we had found it also in the right time period in earth's history and in the right ancient environment. The answer came from 375-million-year-old rocks, formed in ancient streams.
I can do a similar analysis for the wrists, ribs, ears, and other parts of our skeleton—all these features can be traced back to a fish like this. This fossil is just as much a part of our history as the African hominids, such as Australopithecus afarensis, the famous "Lucy." Seeing Lucy, we can understand our history as highly advanced primates. Seeing Tiktaalik is seeing our history as fish.
Some fish, then, had structures like those in a limb. Owen's archetype was not a divine and eternal part of all life. It had a history, and that history was to be found in Devonian age rocks…
Do the facts of our ancient history mean that humans are not special or unique among living creatures? Of course not. In fact, knowing something about the deep origins of humanity only adds to the remarkable fact of our existence: all of our extraordinary capabilities arose from basic components that evolved in ancient fish and other creatures. From common parts came a very unique construction. We are not separate from the rest of the living world; we are part of it down to our bones and, as we will see shortly, even our genes.
His experiments may seem to be a bizarre way to spend the better part of a year, let alone for a young scientist to launch a promising scientific career. Why sharks? Why a form of vitamin A?
To make sense of these experiments, we need to step back and look at what we hope they might explain. What we are really getting at in this chapter is the recipe, written in our DNA, that builds our bodies from a single egg.
Experiment after experiment on creatures as different as mice, sharks, and flies shows us that the lessons of Sonic hedgehog are very general. All appendages, whether they are fins or limbs, are built by similar kinds of genes. What does this mean for … the transition of fish fins into limbs? It means that this great evolutionary transformation did not involve the origin of new DNA: much of the shift likely involved using ancient genes, such as those involved in shark fin development, in new ways to make limbs with fingers and toes.
The power of those moments was something I'll never forget. Here, cracking rocks in the dirt, I was discovering objects that could change the way people think. That juxtaposition between the most child-like, even humbling, activities and one of the great human intellectual aspirations has never been lost on me.
…in teeth, mammary glands, and feathers, we find a similar theme. The biological processes that make these different organs are versions of the same thing. When you see these deep similarities among different organs and bodies, you begin to recognize that the diverse inhabitants of our world are just variations on a theme.
If you want to understand the wiring and plumbing in my building, you have to understand its history, how it was renovated for each new generation of scientists. My head has a long history also, and that history explains complicated nerves like the trigeminal and the facial.
What I've just given you is one of the big tricks for understanding the most complicated cranial nerves and large portions of the head. When you think trigeminal nerve, think first arch. Facial nerve, second arch.
As they looked at embryos, they found something fundamental: all organs in the chicken can be traced to one of three layers of tissue in the developing embryo. These three layers became known as the germ layers. They achieved almost legendary status, which they retain even to this day.
Mangold had discovered a small patch of tissue that was able to direct other cells to form an entire body plan. The tiny, incredibly important patch of tissue containing all this information was to be known as the Organizer… Today, many scientists consider Mangold's work to be the single most important experiment in the history of embryology.
Take the entire 4.5-billion-year history of the earth and scale it down to a single year, with January 1st being the origin of the earth and midnight on December 31st being the present. Until June, the only organisms were single-celled microbes, such as algae, bacteria, and amoebae. The first animal with a head did not appear until October. The first human appears on December 31st. We, like all the animals and plants that have ever lived, are recent crashers at the party of life on earth.
Fossils and the geological record remain a very powerful source of evidence about the past; nothing else reveals the actual environments and transitional structures that existed during the history of life. As we've seen, DNA is an extraordinarily powerful window into life's history and the formation of bodies and organs. Its role is particularly important where the fossil record is silent.
If you compare the odor genes of a mammal with the handful of odor genes in a jawless fish, the "extra" genes in mammals are all variations on a theme… This means that our large number of odor genes arose by many rounds of duplication of the small number of genes present in primitive species.
Our eyes have a history as organs, but so do eyes' constituent parts, the cells and tissues, and so do the genes that make those parts. Once we identify these multiple layers of history in our organs, we understand that we are simply a mosaic of bits and pieces found in virtually everything else on the planet.
Gehring's lab found they could use the mouse gene to trigger the formation of an extra fly eye anywhere: on the back, on a wing, near the mouth. What Gehring had found was a master switch for eye development that was virtually the same in a mouse and a fly. This gene, Pax 6, initiated a complex chain reaction of gene activity that ultimately led to a new fly eye.
As he describes the ear-jaw comparison, his prose departs from the normally staid description of nineteenth-century anatomy to express shock, even wonderment, at this discovery. The conclusion was inescapable: the same gill arch that formed part of the jaw of a reptile formed ear bones in mammals. Reichert proposed a notion that even he could barely believe - that parts of the ears of mammals are the same thing as parts of the jaws of reptiles.
Jellyfish do not have either Pax 6 or Pax 2: they arose before those genes hit the scene. But in the box jellyfish's genes we see something remarkable. The gene that forms the eyes is not Pax 6, as we'd expect, but a sort of mosaic that has the structure of both Pax 6 and Pax 2. In other words, this gene looks like a primitive version of other animals' Pax 6 and Pax 2.
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.
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... 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… or if traits do not have a genetic basis and arise as the result of changes in diet or other environmental conditions.
Our humanity comes at a cost. For the exceptional combination of things we do - talk, think, grasp, and walk on two legs - we pay a price. This is an inevitable result of the tree of life inside us.
These are not esoteric discoveries made on obscure and unimportant creatures. These discoveries on yeast, flies, worms, and, yes, fish tell us about how our own bodies work, the causes of many of the diseases we suffer, and ways we can develop tools to make our lives longer and healthier.
Apollo 8 was a product of the essential optimism that fuels the best science. It exemplifies how the unknown should not be a source of suspicion, fear, or retreat to superstition, but motivation to continue asking questions and seeking answers.
Just as the space program changed the way we look at the moon, paleontology and genetics are changing the way we view ourselves.