The fossil record

Fossils are the preserved remains of the bodies of dead organisms or the remains of the organism's actions — things such as footprints or burrows. The total of all fossils is called the fossil record. The fossil record informs scientists about evolution in several important ways:

i In the past, creatures that we don't find today lived on the planet.

i Not all creatures alive today are represented in the past.

i Through time, the physical complexity of organisms has increased. The earliest organisms that scientists can identify were single celled; now complex creatures exist.

i The earliest forms of life were aquatic; terrestrial forms appeared later.

The fossil record, incomplete though it may be, is a record of change through time. This record gives us clues to the progression of the development of life on Earth: Small single-celled organisms evolved into more complex ones; life started in the oceans and only later moved onto dry land. The fossil record provides a rough draft of the tree of life. (Head to Chapter 9 for a detailed explanation of the role that the tree of life — code word phyolgenetics — plays in evolution.)

A few things about the fossil record stymied Darwin and others in his day, however:

i They seemed to find a lot of older rock that had no fossils and newer rock that had complex life forms, making it seem as though complex life forms appeared suddenly.

i It wasn't clear why certain fossils were found in the locations where they were found — marine fossils on mountaintops, for example.

i Darwin was puzzled by the sudden changes from one type of fossil to the next, when there seemed to be very few, if any, transitional life forms.

The following sections explain what modern science says about these issues.

Conundrum 1: The seemingly sudden appearance of complex life forms

In studying the fossil record, scientists in Darwin's day were limited in a couple of ways that scientists today aren't:

i Their fossil record was even more incomplete than ours today. They didn't have any fossils older than 500 million years.

i They lacked the technology to find microscopic fossils. What Darwin and others perceived as gaps in the fossil record actually weren't gaps at all. Today, scientists have the advantage of a much more thorough search of the planet for older fossils and, more important, far more sophisticated techniques for identifying microscopic fossils in rocks.

The earliest fossils that scientists find are single-celled organisms, which Darwin lacked the ability to see physically. Today, scientists know that life has existed continuously on Earth for about the past 3.5 billion to 4 billion years, and they see the same increase in complexity through time that scientists observed in Darwin's day.

Fossils: Not just rock anymore

Back in Darwin's day, everyone knew that what was so cool about fossils was that biological material had been turned to stone through a process of mineralization. But today, people know something even cooler: Some of the biological material can survive this process and persist for a very long time. Scientists have been able to isolate DNA from organisms, like mammoths and cave bears, that died tens of thousands of years ago.

In retrospect, this feat is not as surprising as it first sounds. DNA is awfully tough stuff; your survival depends on it, after all. Also, techniques for isolating DNA are becoming more and more precise, allowing scientists to work with smaller and smaller quantities.

Even more amazing, scientists recently showed that soft tissue can survive for at least 68 million years inside fossilized Tyrannosaurus rexbone. It hasn't been possible (yet) to isolate DNA from such material, but it has been possible to determine the amino acid sequences of some of the remaining proteins. Tyrannosaurus rex proteins show considerable similarity to the proteins of modern birds — it turns out that T. rexmight've tasted a lot like chicken — not surprising, given the close relationship suggested between dinosaurs and birds by the fossils of Archaeopteryxand other feathered dinosaurs. Now scientists have biochemical evidence supporting the same connection.

Conundrum 2: Marine fossils on mountaintops

Today, scientists understand the process of plate tectonics — the moving around of large chunks of the Earth's surface. The idea that the continents may not be fixed in place was greeted with skepticism as recently as 50 years ago, yet now we know that Earth's crust is composed of a series of plates that move relative to one another, fuse, and break apart, resulting in earthquakes and volcanoes.

What seemed like science fiction a little while ago is now something that science can routinely observe and measure. Submarines can dive to the depths of the ocean where plates are separating so researchers can measure the process. Very accurate markers can be placed in different locations across fault lines and their relative movements can be tracked with satellites and lasers. Scientists know, for example, the rate at which parts of California are moving apart and mountain ranges are pushing higher.

4jtJABCi The fact that continents move explains why fossils turn up in the unlikeliest places: tropical fossils in Antarctica, for example (biologists have every reason to believe that Antarctica was once in the tropics), or seashell fossils on mountaintops (rocks that were once at sea level can be pushed upward over long periods to form mountain ranges). By understanding more about geological processes and time scales, the fossil record is more comprehensible.

Conundrum 3: The seeming lack of transitional forms

Darwin wondered where to find the transitional life forms (consider them the in-between-this-and-that forms). Although we've had more success in finding transitional life forms, today's scientists feel his pain. They are better at knowing where to look and they have more people looking, but they still struggle to find them.

Scientists hypothesize that evolution doesn't occur at a constant rate: It can occur in bursts separated by long periods when not much happens. If the transitional period was brief, the chance that such forms would have been fossilized is even more dicey.

When you think about it, it's really quite amazing that people find fossils at all because the events leading to the fossilization of those wonderfully complete skeletons you read about in the news are quite rare. Conditions have to be just right — leave something in your vegetable crisper for sixty million years, and it's not likely to be enough material to do any sort of analysis on! The organism not only has to die, but it also has to be buried intact and remain undisturbed in conditions hospitable to the mineralization process that preserves the remains. Then, possibly millions of years later, someone stumbles across it and calls in the news cameras.

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