Fossil Evidence for Biological Diversity, Speciation, & Mass Extinction

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  • 0:01 Fossils are Historical Records
  • 1:20 Strata Create A Timeline
  • 2:46 Mass Extinctions & Speciation
  • 4:06 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

One of the best sources of evidence we have for evolution is the fossil record. Fossils provide a wealth of information on how species change over time and for exciting events like mass extinctions and new species formation.

Fossils Are Historical Records

Evolution is generally not something that can be observed within your lifetime. It occurs very, very slowly over thousands or millions of years. Fortunately, we have historical records of changes in populations in the fossil record. It's through this appearance and sequence of fossils within layers of Earth that we can piece together how species evolve and diversify, when new species form, and when catastrophic events occur that wipe out a majority of Earth's species at once.

Charles Darwin is credited with the scientific theory of evolution, and part of what helped him develop this theory was fossils. He traveled all over the world, and during his travels, he observed some interesting things. For example, he found marine fossils up in the mountains of South America. During Darwin's time, these mountains were high above sea level. This means that either these organisms migrated out of the water and up onto these high mountain tops, or that these mountains were once under water themselves.

Since it is quite unlikely that the marine organisms moved themselves so far away from their natural environment, it makes much more sense that this environment was at one time submerged, and marine organisms were right at home!

Strata Create a Timeline

But this only tells us about how Earth evolved over time, not necessarily how organisms or species evolve. What we look to in this case are Earth's strata, or layers of rock. Rock strata help us create a timeline of evolutionary events because they are laid down in order. The oldest strata are found at the bottom, while the youngest strata are found at the top.

So, when we find fossils in various rock strata, we can arrange these fossils along a geological timeline and see how they evolved. For example, it's believed that whales and dolphins evolved from four-legged animals because we have found evidence of these changes in the fossil record. Today, these animals have front flippers that resemble arms, but they do not have back legs. But fossils have been discovered that show how, long ago, ancestors of these animals did in fact have leg bones found only in certain types of mammals.

The fossil record is also useful for showing us the biological variety and diversity between species, known as biodiversity. However, it is not very good at helping us see biodiversity within a species because these differences are often best observed through DNA and organic material. You'll notice that fossils are generally bones or imprints, and this is because organic material breaks down very easily and is therefore rarely preserved in rock.

Mass Extinctions and Speciation

The fossil record also shows us when new species develop: a process known as speciation. Large amounts of speciation tend to occur after mass extinctions, which are when more than half of the species on Earth go extinct over a relatively short geological time period.

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