Fossils are the remains or signs that a particular organism existed at least 10 000 years ago. They include dinosaur bones, mummified remains in deserts, frozen mammoths, insects in amber, petrified wood, moulds, casts, footprints and trails.
Fossils are usually found in sedimentary rocks.
Fossils are rare events, and are often distorted or incomplete.
The structures of fossils resemble many present-day organisms, so palaeontologists infer that they had the same function as that of present-day organisms.
Dating of Fossils is by 3 means:
Law of Superposition states that layers of rocks deeper under the ground are older than those close to the surface.
Index Fossils and Correlation - If layers of the same type of rock containing the same type of distinctive fossils are found in the same location (e.g. on either side of the Grand Canyon), then they are of the same age.
Radioactive Dating - Many elements have isotopes (atoms with the same number of protons but with different numbers of neutrons) that are unstable and radioactive. Half of the original amount can break down over a certain calculated period of time (Half-life), and are used to determine the age of rocks and carbon-containing fossils.
For example, when each half of the original mass of Uranium-238 in rocks breaks down to form Thorium-230, the process takes about 80 000 years. Another example is when half an original mass of Carbon-14 in fossil remains breaks down, the process takes about 5730 years.
Comparison of embryos of terrestrial species (e.g. humans, chickens, pigs) show pharyngeal slits that resemble the gill slits of fish. Also these embryos show tails in the early gestation period, slowly degenerating before the end of the gestation period.
Some species have structures that do not function, but may have functioned in an ancestor (e.g. appendix and wisdom teeth in humans).
Analogous structures are anatomical structures that are found in many unrelated organisms. For example, wings of birds and bees serve the same function of flight. These similar structures do not show signs of an evolutionary relationship.
Homologous structures are structures that are similar (e.g. the five-fingered structure of a human hand compared with the five-fingered structure of a seal, the incisors and molar teeth of various mammals) and also show signs of an evolutionary relationship.
Variation due to Sexual Reproduction
Asexual reproduction produces organisms or new cells identical to the parent organism or cell.
Sexual reproduction where male sperm or pollen combine with the female egg produce similar but not identical offspring.
Some organisms live amicably inside larger organisms, sometimes to the benefit of both (e.g. alga and fungus in lichen).
It may be that eukaryotic cells were formed from a smaller prokaryotic bacterium invading a larger cell. Evidence comes from mitochondria and chloroplasts that both contain DNA.
Specialisation in Algal Colonies
Colonies of many algal types exist, where each individual algal cell is physically attached to other algal cells, but no interaction occurs between cells.
However, Volvox is a 'ball' of algal cells which have one end responsible for movement, while another part of the 'ball' is responsible for reproduction. This may be evidence for the development of multicellular organisms from unicellular organisms.
This is observed example of Natural Selection.
Before the Industrial Revolution in England, a forest-dwelling moth (Biston betularia) grew in 3 shades - light, peppered and dark. As the tree bark was lighter, the most common shade of moth was the peppered one. However, after the soot from the factories darkened the barks of trees in the moth habitat, the most common moth was the dark one. The reason was probably because the light and peppered moths were no longer camouflaged and were being by predators. The dark moths then reproduced, changing the gene frequencies of the population.
Bacterial Resistance to Antibiotics
This is an observed example of Natural Selection.
Bacteria do not become resistant to antibiotics. Either they carry the genetic trait for resistance or they don't.
Before World War II, there were different kinds of bacteria - some which were resistant to antibiotics and some which were not. After the widespread use of antibiotics during and after World War II that killed most of the non-resistant bacteria, the resistant bacteria continued to multiply even more due to lack of competition. Their genetic resistance was passed on to subsequent generations. Hence, many of the bacteria that we encounter today are resistant.
The solution is twofold - firstly, to discourage the widespread use of antibiotics unless necessary, and secondly, to invent new antibiotics.
A mutation is a change in the nucleotide sequence of a DNA molecule, or of a chromosome.
A mutagen is a chemical or a form of radiation that can cause a mutation.
Since DNA controls the synthesis of proteins (e.g. skin collagen, hair keratin, muscle myosin, hormones, enzymes, haemoglobin, antibodies), the proteins formed may be affected.
Mutations can occur naturally. However, they are rare events (about 1 in every 100000 generations).
However, mutations of fruit flies are believed to be harmful, since mutated flies seem to be in few numbers.
One of the bases on which the theory of evolution is built is that chance mutations formed new organisms which could no longer mate with the rest of their species and eventually formed new species. However, since mutations observed in fruit fly experiments are so rare and often cause harm to the individuals, creationists argue that evolution in this way is not plausible.
The DNA strands of different species of primates can be compared when they are unwound and the strands compared for similarities in nucleotide sequence.
DNA of humans and chimpanzees have a much greater similarity (97.6%) than humans and gibbons (94.7%), leading to the idea that humans may have evolved from chimpanzees, and not from gibbons.
Amino Acid Sequencing
When comparing the amino acid sequence of haemoglobin in various primates, the sequences are also more similar for humans and chimpanzees, than for humans and gibbons.
Again, an assumption could be made that humans evolved from chimpanzees.
Did You Know That...? Humans are the only animals that "speak". In the upright human position, the larynx (voice box) is less constricted compared with that of a quadriped primate.