L21. Evidence That Life Changes Over Time

Natural Selection and Evolution

R-report

L21. Evidence That Life Changes Over Time

How do scientists know that species we see today were different long ago—and sometimes look very different from their ancestors?

What this lesson adds (and what you already know)

You’ve learned about genes, chromosomes, and how mutations create variation. Those ideas explain how individual organisms can be different. In this lesson we shift from causes of variation to the bigger question: what evidence shows that whole populations and lineages have changed across many generations? The focus is on the kinds of data scientists use—fossils, body plans, embryos, DNA, and where organisms live—and how a single real example ties them together.

Fossils: snapshots that line up in time

Fossils are remains or traces of organisms preserved in rock layers. Because layers form in order, the fossil record gives a timeline of life. When scientists find fossils in older layers that look different from fossils in younger layers, that sequence is direct evidence of change through time. A powerful case study is early whale evolution: paleontologists found a series of fossils that change gradually from land‑dwelling ancestors to fully aquatic whales. Each fossil preserves details of skull shape, teeth, and limb bones that show a stepwise change.

• Layer order: deeper = older (unless later disturbed). Transition fossils: forms with a mix of ancestral and newer traits (e.g., early whales with hind legs and ear bones adapted for hearing in water). Dating tools (relative layering, radiometric dating) give ages to compare sequences.

Comparative anatomy and embryos: repeated patterns

Different species sometimes share the same basic body parts arranged in similar ways. Homologous structures—like the same set of forelimb bones in bats, humans, and whales—suggest those species inherited a common blueprint and then modified it. Vestigial structures (small or unused organs) point to earlier forms where those parts were useful. Embryonic development can reveal hidden similarities: embryos of very different animals often pass through similar stages, hinting at shared beginnings.

• Homology: same underlying structure, different function (wing, arm, flipper). Vestigial organs: reduced features that made sense in ancestors (e.g., tiny pelvic bones in some whales). Developmental parallels: early embryo stages can show the same tissues forming in similar order.

Molecules and places: DNA and distribution tell the rest of the story

Modern evidence comes from DNA. Closely related species have more similar DNA sequences than distant ones; mapping those similarities produces phylogenies—family trees of life—that match the order seen in fossils and anatomy. Biogeography (where species live) adds context: island species often resemble nearby mainland species more than distant ones, showing descent plus change after isolation. Finally, scientists document small‑scale change in present day (bacteria evolving drug resistance, or insects evolving pesticide resistance). When fossils, anatomy, DNA, geographic patterns, and observed changes all point the same way, the case that life changes over time becomes very strong.

Putting the evidence together

Multiple independent lines of evidence—fossils that preserve ordered change, repeated anatomical patterns, embryonic similarities, DNA comparisons, geographic distributions, and observed small‑scale changes—converge on the same conclusion: life has changed over time. No single piece of evidence stands alone; the strength comes from how they fit together into a coherent story.

This lesson sets up the next idea: having seen that lineages change, we now ask why some variants persist and others disappear. The next lesson on natural selection explains the processes that can make those changes accumulate across generations.