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Evolution isn’t just a guess or an idea - it’s a scientific explanation supported by a wide variety of evidence. It is a foundational concept in all of biology, and part of what makes evolution so robust is that many different fields - from genetics to geology - offer independent lines of evidence, all pointing to the same conclusion: life on earth has evolved and shares common ancestors.
Fossils
Fossils
Fossils are preserved remains or traces of organisms from the past. They give us direct evidence of what ancient organisms looked like, how they changed over time, and when certain features first appeared. Fossils can show transitional forms, like Tiktaalik, which had features of both fish and early land vertebrates.
To understand when fossils lived, scientists use two main dating methods:
Relative dating looks at the position of fossils in layers of rock (strata). Older fossils are found in deeper layers, while younger ones are closer to the surface.
Radiometric dating uses the decay of radioactive isotopes (like carbon-14 or uranium-238) to assign actual numerical ages to rocks and fossils. This method provides more precise time estimates, especially for very old fossils.
Comparative Anatomy
Comparative Anatomy
The structures of different organisms can reveal clues about their evolutionary past.
Homologous Structures
Homologous Structures
Homologous structures have a similar underlying structure but may serve different functions. These similarities suggest the organisms inherited the trait from a common ancestor.
As an example, the forelimbs of humans, dogs, whales, and bats all have the same bone arrangement - but these limbs are used very differently.
Analogous Structures
Analogous Structures
Analogous structures serve similar functions but have different underlying structures. These similarities evolved independently from convergent evolution, not common ancestry.
As an example, bird wings and insect wings are both used to fly, but have very different structures.
Vestigial structures are reduced or unused body parts that were functional in an ancestor - such as the tailbone (coccyx) in humans, leg bones in whales, and non-functional eyes in blind cave fish. These features show that organisms can retain traits from ancestors, even if they no longer serve a purpose.
Homologies can also be found during early development. This is known as comparative embryology, and it involves comparing the embryos of different species to look for patterns that suggest a shared ancestry. For example, the embryos of fish, birds, reptiles, and mammals all develop structures like pharyngeal pouches (which can develop into gills in fish or parts of the jaw, thyroid, and thymus in humans) and tails, even if those features are not present in the adult form. These similarities are often not visible in adults, but they appear during the early stages of growth - suggesting that these species inherited a common developmental plan from a shared ancestor.
Comparative embryology supports the idea that organisms with very different adult forms can still be closely related, because they start from the same basic blueprint. The more similar the developmental pattern of species, likely the more closely related they are.
Molecular Evidence
Molecular Evidence
One of the strongest lines of evidence for evolution comes from molecular biology.
DNA sequences: The more similar two organisms’ DNA is, the more recently they likely shared a common ancestor.
Protein comparisons: Proteins can be looked at for similarity in amino acid sequence, as well. Certain proteins, like cytochrome c or hemoglobin, are highly conserved and can be compared across a wide variety species.
The universal genetic code itself is strong evidence of common ancestry - all living organisms use the same system to translate DNA into proteins.
Biogeography
Biogeography
Biogeography is the study of the geographical distribution of organisms. The patterns of species distribution often reflect evolutionary history.
Species on islands often resemble those on the nearest mainland, even if their environments are very different. Continental drift helps explain why similar fossils appear on continents that are now separated.
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