Course 24.1Lesson 2 of 3
If species change over time, they should leave marks. Fossils in rock layers, shared bones in different animals, the early embryo and the molecules of the cell are all marks of one family tree. This lesson reads each kind of evidence.
Try the code and the simulations beside the lesson.
If all the living things came from earlier forms, we should be able to find proof. Four kinds of evidence are used.
A fossil is the remains or the mark of an organism that lived long ago, usually the hard parts, kept in rock. Sedimentary rocks form in layers, called strata, when mud and sand settle one on another. The old layers lie at the bottom and the young ones lie on top.
Each layer holds the fossils of the living things that died when it was forming. Studying the layers shows three facts.
The order in the layers is the order in which the groups appeared: invertebrates, then fish, then amphibians, then reptiles, then mammals. This is palaeontological evidence.
How old is a fossil? The order of the layers gives a relative age: a deeper layer is older. An absolute age, in years, comes from radioactive dating. Carbon-14 in a living thing is replaced all the time. After death it only decays, and its amount halves every half-life of 5730 years.
This part is not in the NEET syllabus, but A-Level, IB and AP ask it. Fractions left: 1/2 after one half-life, 1/4 after two, 1/8 after three. Carbon-14 can date remains up to about 50000 years old. For older rocks, other elements with longer half-lives are used.
You should see the curve fall by half over each 5730 years, and the fraction fall to one quarter near 11500 years.
Compare the body parts of different animals and plants.
Homologous organs have the same basic structure and the same origin, but they do different jobs. The forelimbs of a whale, a bat, a cheetah and a human all have the same bones: one upper arm bone, two forearm bones, wrist bones, palm bones and finger bones. In the bat the finger bones are very long to hold the wing. In the whale they form a paddle. Plants have them too. The thorn of Bougainvillea and the tendril of Cucurbita are both modified shoots. The hearts and the brains of the vertebrates show the same plan.
Homologous organs show divergent evolution: a common ancestor, then different adaptations. They point to common ancestry.
Analogous organs have different structures and origins, but the same function. Three examples are the wings of a butterfly and of a bird, the eyes of an octopus and of a mammal, and the flippers of a penguin and of a dolphin. The tuber of a potato is a modified stem, and the tuber of a sweet potato is a modified root. Both store food, so they are analogous too.
Analogous organs show convergent evolution: different groups reach the same kind of structure because they live in a similar way. They do not show a close family tie.
Many books and the IB also use vestigial organs, which are small and have lost their old use, as in the human appendix. NCERT does not discuss them. They are not needed for the NEET syllabus.
The embryos of all vertebrates, including the human, form a row of vertical slits on the sides of the neck just behind the head. They are the gill slits, or pharyngeal slits. They stay as gills only in fish. They are not found in the adults of the other groups. Ernst Haeckel took this as proof for his idea that an embryo repeats the adult forms of its ancestors. His words were "ontogeny recapitulates phylogeny".
Karl Ernst von Baer studied the embryos closely and showed that this is not right. Embryos never pass through the adult stages of other animals. What the embryos share are early features, which are then changed. Haeckel's slits still show common ancestry, but his law is not accepted.
Proteins and genes that do the same job in different species are alike. The more alike they are, the closer the relationship. All living things use the same genetic code, the same ATP and the same kind of ribosomes, which points to one common ancestor.
The program below compares a short gene in four species. The sequences are made up for practice. Real studies compare long sequences in the same way.
The program counts the bases that are the same in each pair of sequences.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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