VMBO Year 2 · HAVO/VWO Years 2–3 · Inheritance and evolution · Section 5.5

The history of life on Earth

Fossils

Fossils are preserved remains, impressions or traces of past organisms. Rapid burial by sediment, such as sand or mud, can reduce scavenging and decomposition and improve preservation.

Hard parts often fossilise more readily than soft tissues, although neither fossilisation nor complete prevention of decay is guaranteed.

Fossils show that species have appeared, changed and disappeared. In an undisturbed sequence of sedimentary rocks, newer layers normally lie above older ones.

Geological time

The geological timescale divides Earth's history into units such as eras and periods. It shows their approximate dates and the types of life known from them.

Earth formed about 4,600 million years ago. Evidence of early life dates back at least 3,500 million years, with older evidence still studied and debated. Some school timelines place early life around 3,800 million years ago.

Early microorganisms included photosynthetic forms that eventually increased oxygen in water and the atmosphere. Multicellular life evolved later, with evidence extending well beyond a thousand million years. Early animals appeared hundreds of millions of years ago, initially in water; precise dates remain subjects of research.

Animals on land

Arthropods, including millipede-like animals, were among the early land animals. Later, vertebrate lineages gave rise to amphibians and reptiles.

The Mesozoic Era, beginning about 252 million years ago, included the rise and diversification of dinosaurs. Other reptiles occupied marine environments and the air. Early mammals and, later, birds also evolved during this era.

About 66 million years ago, a major extinction event eliminated non-bird dinosaurs and many other groups. Strong evidence points to a large asteroid impact, with environmental changes and volcanism also relevant to the wider picture.

Birds survived, as did some mammals, and these groups continued to diversify. Early members of the human lineage date back roughly six to seven million years. Our own species, Homo sapiens, appeared about 300,000 years ago.

Relationships between species

Species are related through common ancestry. Comparing DNA, proteins and body structures helps reconstruct their relationships.

Similar underlying structures with different functions, such as the bones of a human arm, a bird wing and a whale flipper, can indicate a shared origin. Their functions changed as lineages evolved in different environments.

Vestigial structures

Some structures have lost or changed much of their ancestral function during evolution. They are called vestigial structures; they are not necessarily entirely useless.

Examples include reduced hind-limb structures in some snakes and human tail vertebrae forming the coccyx. Wisdom teeth are often discussed as a reduced or problematic feature of human evolutionary history.

These structures provide evidence of relationships to ancestors and other species with more fully developed versions.