For early evolutionists, fossils were an important source of evidence for evolution. Fossils show evidence of organisms that were once alive. Most people immediately think of tyrannosaurus rex when they think of fossils, but they come in all sizes, shapes, and species. There are a bunch of different types of fossils, but here are some of the more common examples:
An English dude named William Smith (1769–1839) was one of the first geologists to realize that fossils tend to have a lot in common. He noted that certain fossils always showed up in the same kinds of rock, even if they were found in different locations around England. Using his powers of observation, he also saw that the fossils and the surrounding rock occurred in layers, and these layers usually stacked up in the same order no matter where they were found. Weird, right?
While we can't credit Will Smith with the invention of stratigraphy—the study of layers of rocks and sediments—he was the first person to apply the principles of the science to dating layers. No, he didn't take them to the movies, or whatever passed as first date worthy in the 1700s. Smith found a way to date the layers, as in, tell us how old they were. He realized that, if two different sites had the same rock layer containing the same fossils, they must come from the same time period. Seems like a no brainer, right? But this was the 1700s and even small discoveries were a big deal in the wake of the Dark Ages.
Smith called the predictable layering of fossils the Principle of Faunal Succession, because naming sciency things is awesome. And because the animals (fauna) in the layers always occurred in the same order (succession). Faunal Succession isn't only fun to say, it's also very useful in allowing scientists to correlate layers from different locations.
They could also use their geography context clues to figure out roughly how old a layer was even if surrounding layers were missing (like if there was a cave in the way). Today, we can date sediments using a process called radiometric dating—check out In the Real World: Biotechnology to learn more.
Scientists aren't sure if this fossil is of a plant or an animal, as there are no obvious mouthparts, but it's thought that it lived on the deep ocean floor where it's too dark for photosynthesis. (Source)
Smith, and other scientists of his time made another important observation about fossils. They noted that fossils in the layers closest to the surface (and therefore, the youngest layers) most closely resembled organisms living at the time, while fossils in lower, older layers of rock looked very different from flora and fauna they recognized.
Some of the organisms from the older layers weren't still kicking it on planet earth; they were extinct. Extinction of a species occurs when all members of that species bite the big one. Seems like common knowledge to us, but at the time, this was big news. Scientists made the undeniable conclusion that life changes over time.
So some species were still alive and kicking, while others had gone extinct. Scientists also saw that some of the fossils they found resembled animals or plants of today, but had slight differences, perhaps showing where new species had sprung into existence. These ideas were inconsistent with creationist views and provided strong support for evolution.
Fossils allows us to see the transitional forms animals take on their journey through evolution, morphing through intermediate stages. We can get a glimpse of how one organism evolves into another, or how one species diversifies into many. Check out this info on whale evolution to learn about some of the cool transitional forms that have recently been discovered. These transitional fossils provide direct evidence that life forms change over long periods of time.
Sabretooth squirrels, five-foot penguins, and ants as big as birds. Sounds like a B-rated movie, but Earth used to be a pretty monstrous place. Check out some of the weirdest fossils ever discovered.