FOSSILS are the preserved remains or traces of organisms from the past.
Fossils form when an organism's hard parts (bones, shells, teeth) are buried and slowly replaced by minerals over millions of years. Soft tissue rarely fossilises — so the fossil record is incomplete.
Other types of fossil: trace fossils (footprints, burrows), preserved organisms in amber, ice or tar pits.
How fossils support evolution:
Older rock layers (deeper in the ground) contain simpler, more primitive organisms.
Younger rocks (closer to the surface) contain more complex, more modern-looking organisms.
Transitional fossils show intermediate forms between ancient and modern species — e.g. Tiktaalik (fish-tetrapod transition), Archaeopteryx (dinosaur-bird transition).
Gradual changes in fossil series show how species changed over millions of years.
Limitation: the fossil record is incomplete — most organisms never fossilise, and many fossils are yet to be discovered.
Homologous Structures
HOMOLOGOUS STRUCTURES are body parts that have the SAME UNDERLYING STRUCTURE but are adapted for DIFFERENT FUNCTIONS in different species.
The classic example is the PENTADACTYL LIMB (five-digit limb):
Human arm — for grasping and manipulating objects.
Whale flipper — for swimming.
Bat wing — for flying.
Horse leg — for running.
Frog limb — for swimming and jumping.
All of these limbs have the same arrangement of bones (humerus, radius, ulna, carpals, metacarpals, phalanges) — just modified in shape and proportion for different uses.
This shared fundamental structure strongly suggests they all evolved from a COMMON ANCESTOR that had a basic five-digit limb — the limbs diverged as each lineage adapted to different environments.
If these structures had evolved independently, there would be no reason for them to share the same underlying bone arrangement.
DNA and Molecular Evidence
Modern molecular biology provides some of the strongest evidence for evolution.
DNA COMPARISONS:
All living organisms use the same genetic code (the same triplet of bases codes for the same amino acid, in virtually all species).
This UNIVERSAL GENETIC CODE strongly suggests all life descended from a common ancestor.
The more closely related two species are, the MORE SIMILAR their DNA sequences are.
Example: humans and chimpanzees share approximately 98.7% of their DNA — reflecting our recent common ancestor.
Humans and yeast share about 25% of their genes — both are eukaryotes descended from the same original eukaryotic ancestor.
PROTEIN COMPARISONS:
Similarly, closely related species share more similar protein structures.
Haemoglobin (the oxygen-carrying protein in blood) has a very similar structure in all vertebrates — with differences reflecting evolutionary distance.
VIRUSES and BACTERIA evolving in real time:
Antibiotic resistance in bacteria.
New influenza strains each year.
COVID-19 variants (Alpha, Delta, Omicron).
All demonstrate evolution by natural selection occurring today — observable directly.
⚠️ Common Mistake
The fossil record is INCOMPLETE — most organisms never fossilise. This means gaps in the fossil record are EXPECTED and do not disprove evolution. Students often say 'if there are gaps in the fossil record, evolution can't be proven' — but the overall pattern of fossils strongly supports evolution, even with gaps.
📌 Key Note
Evidence: fossils (older = simpler, gradual change), homologous structures (pentadactyl limb — common ancestor), DNA comparisons (similar species = similar DNA), direct observation (antibiotic resistance, flu evolution). All support evolution.
🎯 Matching Activity — Match the Evidence to What it Shows
Match each piece of evidence to what it demonstrates about evolution. — drag the symbols on the right to match the component names on the left.
Fossil record
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Pentadactyl limb
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DNA comparisons
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Antibiotic resistance
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Universal genetic code
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Bacteria evolving resistance in real time — direct observation of natural selection
All life uses the same DNA code — suggests all organisms descended from a common ancestor
Closely related species have more similar DNA sequences — reflects evolutionary distance
Shows gradual changes in organisms over millions of years — older fossils are simpler
Same bone structure in human, whale, bat and horse limbs — suggests common ancestor
🎯 Test Yourself
Question 1 of 2
1. Why do the pentadactyl (five-digit) limbs of humans, whales and bats support evolution?
2. Why does antibiotic resistance in bacteria provide evidence for evolution?
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