Evolution: How It Works
Billions of years ago, the only living things on Earth were single-celled organisms similar to bacteria. Humans, and all other forms of life, have evolved from these primitive ancestors. Over time, countless changes in the shape, size and function of various parts of the body have given rise to new forms of life--from ants to amoebas, hummingbirds to humans. These evolutionary changes have resulted from changes in DNA and the information it contains.
Examine the Evidence: Similar skulls
Fossils show that long ago, the species roaming the Earth were different from those we see today. Fossils also can show us how species change over time.
Compare the fossil skull of an extinct primate that lived around nine million years ago with the modern gorilla skull. Can you see how they are similar?
Similarities between the skulls suggest that a close relative of this extinct ape probably evolved into the modern gorilla.
In evolution, the fate of an individual depends on which features it has. Each human, for example, inherits a unique combination of thousands of different features, such as eye color, blood type and sensitivity to certain tastes. Some variants, or versions of a feature, may help an organism to survive. Others may be harmful. Still other variants are neutral--neither helpful nor harmful.
In Bicyclus anynana butterflies, a single gene controls the sizes and positions of the spots on the undersides of their wings. Which spot patterns will help a butterfly to survive?
Where Does Variation Come From?
Mutations are random changes in DNA. Some mutations affect a visible feature of the individual, such as the size of a butterfly's wing spots. Others can create resistance to a virus, such as HIV. The same mutation can be harmful, helpful or neutral, depending on the environment.
DNA changes when chromosomes shuffle, or recombine. This happens normally in the process of making a sperm or an egg. As a result, every new child or offspring inherits a unique assortment of DNA from each parent.
Although both children inherited their chromosomes from the same parents, each child got different versions.
The fate of any trait in evolution depends on survival. In the process of natural selection, organisms with an advantageous trait survive and reproduce, passing that trait on to future generations. Those with a harmful trait may die before they reproduce. Which traits are advantageous? It depends on the environment.
Chance Takes Over
When a trait is completely neutral--neither helpful nor harmful--an unpredictable process called drift causes random changes in the frequency of a trait over many generations. If wing spots were neutral, both butterfly types would be equally likely to get eaten. But eventually, one type would become more common than the other, simply by chance.
Slightly different genes
Drift explains many subtle differences between people of different ancestry. For thousands of years, the Basques lived in relative isolation along the mountainous border between France and Spain. As a result of drift, Basques today have one of the highest rates of the Rh-negative blood type in the world.
Bodies of Evidence
Within each human body we can find traces of evolutionary history--evidence that our species has changed over time. Some of the differences between humans and other species can be found in DNA, while others are visible to the naked eye.
DNA on the brain
Certain DNA "signatures" in the human brain show that its structure and inner circuitry changed rapidly during recent human evolution. Studies of brain genes may eventually reveal how humans evolved such large and intelligent brains.
Evolving a voice box
By 350,000 years ago, human ancestors had evolved a descended larynx, the voice box so critical to speech. But some experts say that spoken language probably developed much later--less than 100,000 years ago. The human larynx is an example of an exaptation--something that initially served one function, then later was co-opted for a different purpose.
The larynx sits lower in throat in humans than in chimps--one of several features that enable human speech.
Red-green colorblindness probably originated thousands of years ago. Something about the lifestyle of early Europeans--a greater emphasis on hunting than on foraging perhaps--made color vision less crucial to survival. So, people with color blindness could survive and pass colorblindness on to their children.
Dawn of the dairy
Between 5,000 and 10,000 years ago humans in Europe and northern Africa domesticated cattle and began drinking cow's milk. According to genetic studies, people with a DNA sequence that helped them to digest milk had an advantage during times of famine, so more and more people in those regions inherited this trait.
Oh, my aching...
The lumbar curve in the lower back helps humans maintain their balance as they walk and stand. The curve positions the upper body directly over the feet and hips. But the lumbar curve leaves humans vulnerable to lower back pain and strain. Chimpanzees generally do not walk upright, and thus have straighter spines.
The end of the tail
Look closely at the base of the human spine and you'll find a tailbone--three to five vertebra fused together. It serves no purpose, but reminds us that humans have descended from ancestral animals with tails.