Evidences of evolution
Similar body plans (homologous organs), leftover organs, look-alike embryos and fossils like Archaeopteryx all show that living things share ancestors and changed over time.
Morphological and anatomical evidence: homologous vs analogous organs
Morphology is the study of outer form. Anatomy is the study of inner structure.
Vestigial organs
- Vestigial organs are reduced organs that have no clear use now. They were useful in our ancestors.
- In humans: the vermiform appendix, wisdom teeth (third molars), the tail bone (coccyx) and ear muscles. Also the nictitating membrane, a small fold at the inner corner of the eye.
- Many animals use these parts. A cow moves its ears with ear muscles, and a cat or bird closes its nictitating membrane like a third eyelid.
- Their presence in humans shows that we evolved from ancestors who used them.
- Atavism is the sudden return of an ancestral feature that had disappeared, such as a short tail or thick body hair in a newborn baby. It also points to our ancestry.
Embryological and palaeontological evidence
An embryo is the early stage of an animal growing from a fertilised egg. Early embryos of fish, frog, tortoise, bird and human look very alike. All have gill slits and a tail. Later each grows into its own form. This points to a common ancestor. Ernst Haeckel gave the biogenetic law (recapitulation theory): 'ontogeny recapitulates phylogeny'. This means the growth of an embryo (ontogeny) briefly repeats the evolutionary history of its group (phylogeny). Palaeontological evidence comes from fossils. Older rocks hold simpler fossils; younger rocks hold more complex ones. Some fossils mix the features of two groups. They are called connecting links. Archaeopteryx links reptiles and birds, and lived about 150 million years ago. Living connecting links also exist, such as Peripatus (annelids and arthropods) and the duck-billed platypus (reptiles and mammals).
Archaeopteryx: a connecting link
| Reptile features | Bird features |
|---|---|
| Teeth in the jaws | Body covered with feathers |
| Long tail with many vertebrae (tail bones) | Forelimbs changed into wings |
| Clawed fingers on the wings | Jaws drawn out like a beak |
How to draw Archaeopteryx
- Draw a bird-like body in side view, standing on two legs.
- Draw a small head with long jaws, and small pointed teeth in them. Label teeth (reptile) and beak-like jaws (bird).
- Draw two spread wings with feather lines. Label wings with feathers (bird).
- On the front edge of each wing, draw three small clawed fingers. Label clawed fingers (reptile).
- Draw a long tail with a row of bones down the middle and feathers on both sides. Label long tail with vertebrae (reptile).
- Draw the legs with clawed toes. Write the title: Archaeopteryx: connecting link between reptiles and birds.
Your wisdom teeth and appendix are vestigial organs. They helped our ancestors, who chewed tough raw plant food, but are of little use with today's cooked rice and chapati.
2 marks: “Distinguish between homologous and analogous organs with one example each.” Homologous: same structure and origin, different functions (human arm and bat wing). Analogous: different structure and origin, same function (wings of an insect and a bird).
2 marks: “Why is Archaeopteryx called a connecting link?” It had bird features (feathers, wings, beak-like jaws) and reptile features (teeth, long bony tail, clawed fingers), so it links reptiles and birds.
7 marks: “Explain the evidences of evolution with examples.” Morphological and anatomical (homologous, analogous, vestigial organs), embryological (similar embryos with gill slits, Haeckel), palaeontological (fossils, Archaeopteryx with a labelled diagram).
Homologous = the same home plan, different jobs. Analogous = another plan, the same job.