Introduction to Evidence for Evolution
How do we know that humans and other animals have changed over millions of years? We weren't there to see it happen, but scientists have found "clues" left behind in the Earth. In this chapter, we will explore the evidence for evolution, from tiny bacteria that change in a few days to fossilized ancestors that lived millions of years ago.
Evolution is the gradual change in the inherited characteristics of a population over time. This can eventually lead to the formation of new species.
1. Antibiotic Resistance: Evolution in Action
One of the best pieces of evidence for natural selection (which you can learn more about in the "Darwin and Wallace" chapter) is how bacteria react to antibiotics. Because bacteria reproduce very quickly, we can see evolution happening in "real-time."
How it happens:
1. In a large population of bacteria, some have a random mutation in their DNA that makes them resistant to an antibiotic.
2. When a person takes the antibiotic, the "normal" bacteria are killed, but the resistant bacteria survive.
3. The survivors reproduce and pass on the gene for resistance to their offspring.
4. Over time, the resistant strain becomes more common. This is survival of the fittest in action!
Key Takeaway: Antibiotic resistance provides evidence for evolution because it shows that a population's characteristics can change over generations due to environmental pressure.
2. Evidence from Human Evolution: Fossils
Scientists have found fossils that show how humans have changed over millions of years. These ancestors are called hominids. You need to know about three specific examples that show a clear trend in development:
Ardi (Ardipithecus ramidus)
Found in Ethiopia, Ardi lived about \(4.4\) million years ago. Her features are a mix of ape-like and human-like:
• Ape-like: Long arms and a "big toe" that stuck out for climbing trees.
• Human-like: The structure of her leg bones suggests she could walk upright.
Lucy (Australopithecus afarensis)
Found in Ethiopia, Lucy lived about \(3.2\) million years ago. She was more human-like than Ardi:
• She had arched feet (better for walking than climbing).
• Her brain size was slightly larger than Ardi’s, but still small.
• Her leg and hip bones show she walked upright much more efficiently.
Leakey’s Discovery (Homo erectus)
In 1984, Richard Leakey and his team found fossils in Kenya from \(1.6\) million years ago. One famous find is "Turkana Boy":
• He was much more human-like than Lucy.
• He had a much larger brain capacity.
• His leg and arm structure were very similar to modern humans, showing he was well-adapted to walking and running long distances.
Don't worry if this seems tricky at first! Just remember: as we move from Ardi to Lucy to Leakey’s fossils, the brain size increases and the skeletons become better at walking upright.
3. Evidence from Stone Tools
As human ancestors evolved, they also became more intelligent. We can see this by looking at the stone tools they left behind. These tools became more complex over time:
1. Simple Stone Tools (Older): Used by Homo habilis (\(2.5\) to \(1.5\) million years ago). These were simple pebble tools made by hitting rocks together to create a sharp edge for scraping or cutting meat.
2. More Complex Tools (Mid-range): Used by Homo erectus (\(2\) to \(0.3\) million years ago). These include "hand axes" which were shaped more carefully and could be used for hunting or digging.
3. Advanced Tools (Newer): Used by Homo sapiens (from about \(200,000\) years ago). Tools became much pointier, thinner, and more specialized, such as arrowheads, needles, and fish hooks.
How do we date these tools?
Scientists use two main methods:
• Stratigraphy: Looking at the layers of rock. Tools found in deeper layers are generally older than those found near the surface.
• Carbon-14 Dating: If the tool is found with organic material (like a wooden handle or near a bone), scientists can use carbon dating to find the age of that material.
4. The Pentadactyl Limb (Biology Only)
A pentadactyl limb is a limb that ends in five digits (fingers or toes). You can see this bone structure in humans, bats, whales, and cats.
Why is this evidence for evolution?
Even though a bat uses its limb to fly, a whale to swim, and a human to pick things up, the underlying bone structure is almost identical. This suggests that all these animals evolved from a common ancestor that had a five-fingered limb. If they had evolved independently, it is very unlikely they would all have the exact same bone arrangement!
Quick Review: "Penta" = five, "Dactyl" = fingers. Think of a pentagon having five sides!
5. Genetic Analysis and the Three Domains
In the past, we classified all living things into five kingdoms (Animals, Plants, Fungi, Protists, and Prokaryotes). However, as our technology improved, we started looking at DNA and RNA sequences.
Scientists (led by Carl Woese) discovered that some organisms in the "Prokaryote" kingdom were actually very different from each other genetically. This led to a new system called the Three Domains:
1. Archaea: Look like bacteria but have different DNA sequences. Often found in extreme places like hot springs.
2. Bacteria: "True" bacteria.
3. Eukarya: Includes everything with a nucleus (Plants, Animals, Fungi, and Protists).
Key Takeaway: Genetic analysis is a more accurate way to show how closely related organisms are than just looking at their physical appearance.
Common Mistakes to Avoid
• Misconception: "Individuals evolve during their lifetime."
Correction: Individuals do NOT evolve; populations evolve over many generations.
• Misconception: "Bacteria 'try' to become resistant to antibiotics."
Correction: Resistance happens by random mutation. The antibiotic just selects which bacteria survive.
• Exam Tip: If asked about tool dating, remember that dating the rock layer (stratigraphy) is the most common way to estimate the age of a stone tool.
Summary Checklist
• Can you explain how antibiotic resistance supports Darwin’s theory?
• Can you name the three hominid fossil examples (Ardi, Lucy, Leakey) and describe how they changed?
• Do you understand that stone tools became more complex over time?
• (Biology Only) Can you explain how the pentadactyl limb proves we share a common ancestor?
• Can you state why we moved from Five Kingdoms to Three Domains? (Answer: Genetic/DNA analysis).