Directions of evolution, origin of life and humans

Macroevolution is the long-term evolutionary process that produces groups above the species level (genera, families, classes, phyla). It is driven by the same factors as microevolution, but over far longer periods of time. The exam often asks you to distinguish the directions of evolution and to know the hypotheses on the origin of life, the order of the geological eras and the stages of human evolution.

§1. Biological progress and regress

A. N. Severtsov introduced two concepts for judging the outcome of evolution:

• Biological progress — the success of a group: rising numbers of individuals, an expanding range, and new species and subspecies appearing within the group. Insects, flowering plants and rodents are in a state of biological progress today.
• Biological regress — the opposite: numbers fall, the range shrinks, species diversity declines, and the group is threatened with extinction (many groups of ferns and their allies, large predators).

§2. Directions of evolution

• Aromorphosis (arogenesis) — a major change that raises the level of organisation and the overall intensity of life processes. It allows new environments to be colonised and leads to large groups (classes, phyla). Examples: photosynthesis, the eukaryotic cell, multicellularity, the sexual process; in plants — tissues and organs, the seed, the flower and double fertilisation; in animals — the notochord, jaws, lungs, the four-chambered heart, warm-bloodedness, the placenta.
• Idioadaptation (allogenesis) — a specific adaptation to particular conditions that does not raise the level of organisation. It leads to small groups (species, genera). Examples: protective colouration, the variety of beak and foot shapes in birds, the flat body of rays, the spines of desert plants, cactus leaves turned into spines.
• General degeneration (catagenesis) — simplification of organisation, usually linked to a parasitic or sedentary way of life: tapeworms have no digestive system and poorly developed sense organs but powerful reproductive organs; dodder has no roots or leaves and almost no chlorophyll; cave animals have reduced eyes.

Patterns of group evolution: divergence — descendants of one ancestor become increasingly different (Darwin’s finches); convergence — unrelated organisms develop similar features in similar conditions (the streamlined bodies of sharks and dolphins); parallelism — related groups develop similar features independently.

Worked example

Classify by direction of evolution: 1) the tough eggshell of reptiles; 2) the hooked beak of an eagle; 3) the loss of leaves in dodder; 4) warm-bloodedness in birds; 5) fat in a camel’s hump.

1. The eggshell made reproduction on land possible — aromorphosis.
2. Beak shape is a specific adaptation — idioadaptation.
3. Simplification of a parasitic plant — degeneration.
4. Warm-bloodedness — aromorphosis.
5. A specific adaptation to the desert — idioadaptation.

§3. The origin of life and Earth’s history

Main views: creationism (divine creation), spontaneous generation, panspermia (life came from space), and the biochemical evolution hypothesis. The idea that life arises spontaneously from non-living matter was disproved by F. Redi (experiment with meat and flies) and L. Pasteur (experiment with a swan-necked flask).

Stages of the hypothesis of A. I. Oparin (1924) and J. Haldane:

1. In the primary atmosphere (methane, ammonia, hydrogen, water vapour; almost no free oxygen), lightning, ultraviolet light and volcanic heat produced simple organic substances abiogenically — amino acids, sugars, nitrogenous bases. S. Miller (1953) recreated these conditions in the laboratory and obtained amino acids.
2. Accumulating in the primary ocean (the "primordial soup"), they formed biopolymers — proteins and nucleic acids.
3. Biopolymers formed coacervate droplets — systems separated from their surroundings that absorb substances.
4. Once a membrane and self-replication (nucleic acids) appeared, the first cells (protobionts) arose.

The first organisms were anaerobic heterotrophic prokaryotes. Then photosynthetic organisms (cyanobacteria) appeared, the atmosphere became rich in oxygen, the ozone layer formed, and life could move onto land.

• Archaean — prokaryotes; photosynthesis appears.
• Proterozoic — eukaryotes, multicellular organisms, algae, invertebrates.
• Palaeozoic — plants colonise land (psilophytes), fish, the first amphibians and reptiles; giant forests of ferns and their allies (coal deposits).
• Mesozoic — the "age of reptiles" (dinosaurs), gymnosperms flourish; the first mammals and birds, and later flowering plants.
• Cenozoic — dominance of mammals, birds, flowering plants and insects; humans appear.

§4. Anthropogenesis — the origin of humans

Humans in the classification system: kingdom Animals → phylum Chordates → subphylum Vertebrates → class Mammals → order Primates → family Hominids → genus Homo → species Homo sapiens.

Evidence of human kinship with animals: gill slits and a tail in the embryo; vestigial organs and atavisms; similarity with the great apes in blood groups, proteins and chromosome structure.

Stages of evolution:

• Australopithecines — walked on two legs and used ready-made objects as tools.
• Handy man (Homo habilis) — the first to make stone tools.
• Upright man (Homo erectus: Pithecanthropus, Sinanthropus) — used fire and spread out of Africa to other continents.
• Neanderthals (ancient people) — complex tools, burial of the dead. In Uzbekistan, the skeleton of a Neanderthal child was found in the Teshik-Tash cave in Surkhandarya (A. P. Okladnikov, 1938). Modern science regards Neanderthals not as our direct ancestors but as a closely related parallel branch.
• Cro-Magnons — people of modern appearance (Homo sapiens): speech, art (cave paintings), complex social life.

Modern people are divided into three major races (Caucasoid, Mongoloid, Negro-Australoid), but all belong to one species: they interbreed freely and have fertile offspring. Dividing races into "higher" and "lower" (racism) has no scientific basis.

§5. How Earth’s history is measured: fossils and radiometric dating

The main record of macroevolution is fossils: petrified skeletons, shells, imprints, insects in amber. Sedimentary rocks lie in layers, and normally lower layers are older than upper ones. This gives relative age — which organisms lived earlier and which later.

Absolute age is found from the decay of radioactive isotopes. Each isotope has a half-life (T½) — the time in which half of its atoms decay.

• Radiocarbon ¹⁴C (T½ ≈ 5730 years) — an organism takes it in while alive and the amount falls after death; it is used for remains up to about 50 000 years old (bone, wood, charcoal).
• For ancient rocks, long-lived isotopes are used (potassium–argon and uranium–lead methods).

Key dates (approximate):

Era boundaries are marked by mass extinctions: at the end of the Palaeozoic (Permian period) most marine species disappeared, and at the end of the Mesozoic the dinosaurs (apart from birds) died out. Afterwards other groups filled the vacant niches in rapid adaptive radiation (divergence) — for example, mammals in the Cenozoic.

Worked example

Charcoal from an ancient campfire contains 1/8 as much ¹⁴C as a living tree. How long ago did the fire burn (T½ = 5730 years)?

1. 1/8 = (1/2)³ → 3 half-lives have passed.
2. t = 3 × 5730 = 17 190 years.
3. Answer: about 17 000 years ago.

Worked example

Place these events by era: a) the first birds; b) psilophytes colonising land; c) the first eukaryotes; d) the appearance of humans; e) the first amphibians.

1. Eukaryotes — Proterozoic (c).
2. Psilophytes and the first amphibians — Palaeozoic (b, e); psilophytes came before amphibians.
3. The first birds — Mesozoic (a); humans — Cenozoic (d).
4. Answer: c → b → e → a → d.

Check yourself

1. Name the criteria of biological progress.

2. What kind of change is the four-chambered heart?

3. What is the similarity in body shape between a shark and a dolphin called?

4. What did Miller’s experiment show?

5. In which era did dinosaurs dominate?

6. Who first made stone tools?

7. A specimen retains 1/4 of its original ¹⁴C. Roughly how old is it?

8. About how many million years ago did the Cenozoic era begin?

Common mistakes

• Calling protective colouration an aromorphosis — it is an idioadaptation.
• Equating degeneration with biological regress — parasites often thrive.
• Getting the order of eras wrong, for example placing the Mesozoic before the Palaeozoic.
• Describing the first organisms as aerobic or autotrophic — they were anaerobic heterotrophs.
• Saying humans descended from apes — humans and modern great apes share a common ancestor.
• Treating races as different species — all races belong to the single species Homo sapiens.