The Groupings of Life

From the book 'Your Inner Fish: A Journey into the 3.5 Billion-Year History of the Human Body' P. 7 - 9.

"That a column of rocks has a progression of fossil species probably comes as no surprise. Less obvious is that we can make detailed predictions about what the species in each layer might actually look like by comparing them with species of animals that are alive today; this information helps us to predict the kinds of fossils we find in ancient rock layers. In fact, the fossil sequences in the world's rocks can be predicted by comparing ourselves with the animals at our local zoo or aquarium.

How can a walk through the zoo help us predict where we should look in the rocks to find 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. All the living things can be organized and arranged like a set of Russian nesting dolls, with smaller groups of animals comprised in bigger groups of animals. When we do this, we discover something very fundamental about nature.

Every species in the zoo and aquarium has a head and two eyes. Call these species 'Everythings.' A subset of the creatures with a head and two eyes has limbs. Call the limbed species 'Everythings with limbs.' A subset of these headed and limbed creatures has a huge brain, walks on two feet, and speaks. That subset is us, humans. We could, of course, use this way of categorizing things to make many more subsets, but even this threefold division has predictive power.

The fossils inside the rocks of the world generally follow this order, and we can put it to use in designing new expeditions. To use the example above, the first member of the group 'Everythings,' a creature with a head and two eyes, is found in the fossil record well before the first 'Everythings with limbs.' More precisely, the first fish (a card-carrying member of the 'Everythings') appears before the first amphibian(an 'Everything with limbs"). Obviously, we refine this by looking at more kinds of animals and many more characteristics that groups of them share, as well as by assessing the actual age of rocks themselves.

...

The order of fossils in the world's rocks is powerful evidence of our connections to the rest of life. If, digging in 600-million-year-old rocks, we found the earliest jellyfish lying next to the skeleton of a woodchuck, then we would have to rewrite our texts. That woodchuck would have appeared earlier in the fossil record than the first mammal, reptile, or even fish--before even the first worm. Moreover, our ancient woodchuck would tell us that much of what we think we know about the history of the earth and life on it is wrong. Despite more than 150 years of people looking for fossils--on every continent of earth and in virtually every rock layer that is accessible--this observation has never been made."