Evolution — Evidence and Human Evolution

Evidence for evolution

Evolutionary theory is supported by multiple, independent lines of evidence that all converge on the same conclusion — that living species share common ancestry and have changed over time.

The fossil record — fossils show a chronological sequence of change, including transitional forms that share features of two different groups (e.g. Archaeopteryx, showing both reptilian and bird-like features), demonstrating gradual change over millions of years.
Comparative anatomy — homologous structures (same basic structure, inherited from a common ancestor, but adapted for different functions — e.g. the forelimb bones of a human, whale, and bat) provide strong evidence of shared ancestry. Analogous structures (similar function but different origin, e.g. the wings of a bird and an insect) arise from convergent evolution, not common ancestry.
Molecular/DNA evidence — species that are more closely related share a greater percentage of similar DNA and protein sequences. The degree of genetic similarity between species reflects how recently they shared a common ancestor.
Biogeography — the geographical distribution of species (e.g. unique marsupials in Australia, isolated for millions of years) reflects patterns of migration, isolation, and independent evolution.

Homologous vs analogous: homologous structures = same origin, different function (evidence of common ancestry). Analogous structures = different origin, similar function (evidence of convergent evolution, not common ancestry).

Human evolution — the hominin lineage

Fossil evidence shows a lineage of hominins (the group including modern humans and our extinct close relatives) evolving in Africa over millions of years, showing clear trends over time.

Trend	Change over time
Brain size	Steady increase from early hominins to modern humans
Posture	Increasing bipedalism (walking upright on two legs)
Tool use	From simple stone tools to increasingly complex technology
Skull/jaw	Reduction in jaw size and brow ridge prominence

Key stages in the hominin lineage include (in order): Australopithecus (small-brained, bipedal) → Homo habilis ("handy man", first stone tool use) → Homo erectus (larger brain, controlled use of fire, first to migrate out of Africa) → Homo sapiens (modern humans, complex language, art, and culture).

South Africa has produced some of the world's most important Australopithecus africanus fossils: the Taung Child, Mrs Ples, and Little Foot — all discovered at South African sites and central to our understanding of early hominin evolution.

Bipedalism — walking on two legs

Several specific skeletal features distinguish bipedal hominins from quadrupedal apes, and each has a direct mechanical reason for supporting upright walking:

Feature	Significance for bipedalism
Foramen magnum (forward position)	Positioned further forward and underneath the skull (rather than at the back, as in apes), allowing the vertebral column to attach directly below it — this lets the head balance upright on top of the spine without needing large neck muscles to hold it up
S-shaped spine	The curved (S-shaped) spine better distributes the weight of the upper body, absorbs shock during walking, and gives flexibility — a straight spine (as in apes) is suited to quadrupedal movement, not upright balance
Short, wide pelvis	A short, bowl-shaped pelvis supports the weight of the upper body directly above the hip joints and anchors the muscles needed for upright walking; the narrow, elongated ape pelvis is suited to quadrupedal movement instead

The "Out of Africa" hypothesis

Genetic and fossil evidence strongly supports the "Out of Africa" hypothesis — that Homo sapiens originated in Africa roughly 200,000 years ago, then migrated outward in waves to populate the rest of the world, gradually replacing other hominin species (such as Neanderthals) that had migrated out of Africa earlier.

Note: human evolution is not a single straight "ladder" — several hominin species often existed at the same time, and the lineage is better pictured as a branching bush than a straight line.

Species and reproductive isolation

A biological species is a group of organisms with similar characteristics that can interbreed to produce fertile offspring. When populations of the same ancestral species can no longer interbreed, they have become separate species — this happens through reproductive isolation mechanisms:

Geographic isolation — a physical barrier (e.g. a mountain range, ocean, or continental drift) separates populations so they cannot meet to breed.
Temporal isolation — populations breed at different times (e.g. different seasons, times of day, or years), so they never mate even if they occupy the same area.
Behavioural isolation — differences in courtship behaviour or mating rituals mean individuals from different populations do not recognise each other as potential mates.
Mechanical/structural isolation — physical differences (e.g. incompatible reproductive structures) prevent mating from being physically possible.
Hybrid infertility — even if mating occurs between two populations, any offspring produced are infertile (e.g. a mule), so no gene flow continues between the populations.

Exam tip: if a question gives you a scenario (e.g. two related species breeding in different months, or living on opposite sides of an ocean), name the SPECIFIC mechanism the scenario describes (temporal or geographic), not just "reproductive isolation" in general.

Answer in your exercise book.

Question 1 · (4 marks)

Name the FOUR main lines of evidence for evolution, with a brief description of each.

Question 2 · (3 marks)

Distinguish between homologous and analogous structures, giving an example of each.

Question 3 · (4 marks)

Name, in order, the FOUR hominin stages discussed in this lesson, from earliest to most recent.

Question 4 · (3 marks)

Describe THREE trends seen across hominin evolution (e.g. changes in brain size, posture, and tool use).

Question 5 · (3 marks)

Explain the "Out of Africa" hypothesis for the origin and spread of modern humans.

Question 6 · (5 marks)

The table shows approximate brain volume and age (using the midpoint of each species' age range) for four hominin species:

Species	Approx. age (million years ago)	Brain volume (cm³)
Australopithecus	3.0	450
Homo habilis	2.0	625
Homo erectus	1.0	1000
Homo sapiens	0.1	1350

(a) Calculate the average rate of increase in brain volume, in cm³ per million years, between Australopithecus and Homo habilis.
(b) Calculate the average rate of increase in brain volume between Homo erectus and Homo sapiens.
(c) Compare your answers to (a) and (b). Was the pace of brain volume increase constant across hominin evolution, or did it speed up/slow down? Suggest what this might indicate about selective pressures on brain size in more recent hominins.

Evolution — Evidence and Human Evolution

Evidence for evolution

Human evolution — the hominin lineage

Bipedalism — walking on two legs

The "Out of Africa" hypothesis

Species and reproductive isolation

Hominin Timeline Explorer