There are four major categories of evidence for evolution
fossil evidence,
anatomical evidence, embryological evidence and biochemical evidence
Let's look at fossil evidence first
Fossils are often used as evidence for evolution. They're the traces or remains of dead organisms that lived long ago
Sedimentary rock contains the most fossils and is formed from mud, sand, silt and other fine particles
To make a fossil,
first the animal needs to die and be quickly buried under the mud or silt so that no other critters will be able to break
down the dead animal
Over time,
the minerals in the mud and silt will replace the tissue of the animal and will later reveal a mold or cast of the
long-dead organism
The law of superposition is that older layers are on the bottom and younger layers of rock are on top
This allows us to relatively figure out the age of different organisms or fossils that we find in layers of rock
So an organism found below another organism would be older than the one on top
So letter z is older than letter x in this case
Absolute dating places a numerical age range using radiometric dating,
luminescence dating,
dendrochronology and amino acid dating
Some scientists prefer not to call it absolute
but chronometric or calendar dating instead because the word absolute makes it sound much more certain and precise than it really is
It's still an estimation, like the relative dating of rocks based on the law of superposition
But it has a number to it
For example,
in dendrochronology
scientists count the rings in a tree to estimate the number of years that have gone by
Differences in growth through the seasons create a ring, and it generally takes one year, but there can be some variation
Transitional species are intermediate species which may be in the fossil record or could be missing from the fossil record
It's proposed that there would be evidence of species that were in the middle evolution steps from one species to another
Archaeopteryx pictured here is used to provide a link between the theory that birds evolved from reptiles
Now we're going to start looking at the anatomical evidence for evolution
Homologous structures help scientists find relatedness among species
There are anatomical structures in different species that originated by heredity from a structure in the most recent common ancestor of the species
For example, if you look at the bone structure of a pterodactyl, a bat and a bird you'll see some basic similarities
This suggests they had a common ancestor with a similar forearm structure perhaps very long ago
but if you look at how they use their forearms for flight
they're very different
The Pterodactyl has one long finger that spans a large section of the wing. The bat has a wing between its fingers and
the bird has feathers all along the forearm. These are analogous structures because even though they have closely related functions,
they do not have a common ancestor for that function. Each of these species evolved their ability to fly from a different ancestral line
Vestigial structures are another piece of evidence because the structures seem to serve no function,
but resemble structures with functional roles in other organisms. An example that's commonly used is the human appendix
The appendix was thought to be a shrunken remain of the cecum that can be found in many kinds of herbivores
The idea is that it's a remnant of evolution yet to be truly removed
In 2013 however, scientists refuted the relationship between cecum size and appendix presence
The appendix is actually a housing complex for mutualistic bacteria. That aid in digestion for many species including humans
Embryology is the study of the development of embryos. The more closely related species would be similar in their earliest stages of development
These exaggerated drawings by Ernst Haeckel were used to show similarities between early stages of an embryo among different species including
salamanders, pigs and humans. And while they're not truly accurate they were some of the first attempts at using embryology as evidence for evolution
Today the stages are studied at the same point in their evolution
unlike these images
Lastly when we read DNA in amino acids to find a quantitative way to organize evolutionary ancestry, we're using a molecular clock
Mutations happen at a steady rate and the more mutations there are mean a more distant relationship between the species
Fewer mutations means a more recent common
ancestor. So based on the amino acid differences between human hemoglobin and gorilla gemoglobin,
gorillas are the closest relative to humans on this list
And lampreys are the farthest.
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