This passage comes from a book titled Finding Your Inner Fish by Neil Shubin (© 2008 Neil Shubin).
- 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 the species of
- animals that are alive today; this
- information helps us to predict the
- kinds of fossils we will 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 important fossils?
- A zoo offers a great variety of
- creatures that are all distinct in many
- ways. To pull off our prediction,
- we need to focus on what 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 the
- 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 designed
- 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
- “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 the
- rocks themselves.
- Besides helping us refine the
- groupings of life, hundreds of years
- of fossil collection have produced a
- vast library, or catalogue, of the ages
- of the earth and the life on it. We can
- now identify general time periods
- when major changes occurred.
- Interested in the origin of mammals?
- Go to rocks from the period called
- the Early Mesozoic; geochemistry
- tells us that these rocks are likely
- about 210 million years old.
- Interested in the origin of primates?
- Go higher in the rock column, to the
- Cretaceous period, where rocks are
- about 80 million years old.
- 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.