January 2007

Why is evidence important in science?

Levinton: The outside world, to physical scientists, is the way you gather information. There may be controversy in the way you interpret this information, but evidence is what you collect from the outside world. It has two important roles:

Artist’s impression of five-eyed Opabinia at the sea bottom. The animal genus was found in Cambrian fossil deposits. Author: Arthur Weasley.

• There are facts that command explanation. A simple example is Why does the sun rise daily?

• It allows us to test hypotheses, or ideas that explain the facts. An example of a hypothesis is that the sun seems to rise every day because of Earth’s rotation.

Observation and hypothesis are both important. Accidental discovery is crucial. People finding fossils has gone on for hundreds of years. But using fossil evidence to test a hypothesis is what ensures that science will present accurate statements, research, and theories.

Some people do not understand the difference between “theory” as used in science and “theory” as used in general conversation. So, would you clarify the concept?

Levinton: In general conversation, people might say “I have a theory” when they mean they have an idea or are making an assumption. In science, a theory is not based on speculation. There are many steps to take before a theory is established.

• A hypothesis is a testable statement explaining observations about phenomena occurring in the natural world.
• A theory is a hypothesis or group of related hypotheses that have been repeatedly tested and which scientists generally agree conform to all known data/observations or a major set of observations about the world.

The Cambrian explosion is an important event in Earth’s history. What have we learned about it so far?

Levinton: The Cambrian explosion is a brief time in the Early Cambrian when most major groups of animals that have bilateral symmetry first appear in the fossil record. A bilateral animal is one whose body plan is such that it has two mirror-image halves. Modern examples are lobsters, people, dogs, and butterflies. The event is referred to as an “explosion” because a rich diversity of species appeared in a relatively short amount of time.

The hypothesis is that all these animal groups arose from a common ancestor and diverged at or near the beginning of the Cambrian period, which spans 543 million to 490 million years ago. Evidence is growing to support this hypothesis, at least from evidence derived from fossil occurrences. After that period, very few additional animal phyla, or large animal categories, arose.

A trilobite (Parkaspis decamera) from the Cambrian Period found in the Burgess Shale, Canada.
Image © Oklahoma University, Photographer Albert Copley; Source: Earth Science World Image Bank

How do we know all of this happened?

Levinton: We know it from evidence. There are two things we need to know:

• You have to have a series of rocks from natural sites that are dated scientifically. Rocks are dated by their relative location and other methods but also by radiometric dating. Radiometric dating involves the use of radioactive isotope series that have half-lives up to many billions of years, such as uranium/lead.

• The occurrence of the fossils. What we know now is that many of the animal groups go back in time but not past the Cambrian period.

Fossils are not always preserved perfectly. Sometimes you will come across a lack of good preservation factors for 200 million years, say, for an appropriate fossil to occur. Evidence shows that the rocks before the explosion were suitable for fossils to be formed but most of the Cambrian animals do not appear in these rocks. Other groups are found before the Cambrian, but not the bilaterian groups participating in the Cambrian explosion, except for a few still controversial specimens.

So the date of the rock in which a fossil is found is the date of the fossil. However, it’s possible that a rock can be transported by natural events, for example, eroded out of a rock, transported downstream by a strong current, and deposited somewhere else. Scientists have to be careful about that possibility. Even the famous Burgess Shale in the Rocky Mountains of Canada, where Cambrian fossils were found, may consist of some animal fossils that were transported a few thousand yards. Scientists have to calibrate the data to make sure they are dated correctly.

Can molecular clocks determine the lineage of a fossil from such distant times as the Cambrian?

Levinton: You can never date rocks with molecular clocks, but you can ask certain questions. If you have two organisms and the DNA sequence of a certain type of molecule that evolved slowly enough so that you can see the difference in DNA sequence in the two organisms, you can go back in time to see when they diverged on the tree of life. However, you must have a way to calibrate the difference in DNA sequence against an absolute time scale.

Molecular clocks are not that accurate going back to such distant periods as the Cambrian, for several reasons:

• There are different ways you can make an analysis, but the calibration points are not that abundant. Let’s say you have a 400-million-year-old fossil and another one that arose 430 million years ago. But which age do you use in your evolutionary calculations? It could be a source of error.

• There is also a lot of variation in rates of evolution and that has to be compensated for. There are statistical challenges here.

When looking at shorter spans of time, say 5 to 10 million years before the present, scientists are a lot more confident. There’s a lot more to be learned about molecular clocks to use them accurately for older times such as the Cambrian explosion.

Did the Cambrian explosion happen because it followed an extinction event?

Levinton: Maybe. There are groups of organisms that seem to have some major overturns just before the Cambrian. There are also some physical changes on Earth that are well known, but no one can pinpoint the time. There’s an idea, bolstered by data, that the whole of the Earth was covered by ice, which suggests that the oceans were anoxic, that is, life in the oceans was nonexistent. That would have been an extinction event, which, as history shows, is often followed by a burst of new species. But it would be difficult to connect this possible extinction event to the Cambrian explosion. There are other changes that occurred just before the Cambrian, but these include everything from a lowering of ocean temperature to an increase in oxygen in the atmosphere. There are too many variables that are too poorly timed to help us very much at this time.

Why is the Cambrian explosion so pivotal as an example of macroevolution?

Levinton: Macroevolution is about natural processes on a grand scale of geological time, such as origins and extinctions. The Cambrian explosion is the mother of all animal radiations. All the major body plans—for example, arthropods, brachiopods, and so on—they all arose in a short window of time, if the current fossil record is to be taken at face value. Scientists are still searching for evidence to add to the wealth of knowledge about this period so we can all agree that this hypothesis is absolutely accurate. If it proves to be absolutely true, it means that most of life’s diversity pretty much started then. It’s the moment of animal evolution’s creativity.

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