History of Life on Earth

Table of Contents
1. Life's history
2. The geological timescale
3. The Cambrian explosion
4. Mass extinctions
5. Fossils and missing links
6. Methods used to date fossils
7. Southern African evidence for key events
8. Fossil tourism and opportunities
9. Summary
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The Earth preserves a long record of life through fossils and rock layers. Life began billions of years ago and has changed repeatedly through evolution, extinction and environmental shifts. Fossils, geological events and biological evidence together allow scientists to reconstruct a timeline of major developments in the history of life. Southern Africa provides many important fossil sites that illustrate key events in this history.

Life's history

All present-day organisms share common characteristics that point to common ancestry. Over billions of years, species have diverged and diversified by the process of evolution, driven by variation, natural selection and environmental change.

The broad pattern of major events in the history of life on Earth is:

• Origin of life: simple, single-celled organisms (bacteria and archaea) appeared about 3.5 billion years ago.
• Oxygenic photosynthesis: photosynthetic organisms (notably cyanobacteria) evolved early and produced oxygen; this process led to a progressive oxygenation of the atmosphere (the Great Oxidation Event occurred about 2.4 billion years ago).
• Eukaryotes: cells with a nucleus and internal membrane compartments arose, roughly 1.9 billion years ago.
• Multicellularity: organisms composed of many cells began to appear between about 1.7 and 1.2 billion years ago, with more complex multicellular forms becoming common later.
• Algae and early plants: multicellular algae existed by about 1 200 million years (1.2 billion years) ago; true land plants with vascular tissues evolved much later.
• Invertebrates and the Cambrian expansion: simple invertebrates existed before the Cambrian; during the Cambrian period (beginning about 543 million years ago in older conventions) many modern animal phyla first appear in the fossil record.
• Vertebrates and terrestrial vertebrates: fishes, amphibians, reptiles, birds and mammals appeared in that general order over the Paleozoic and Mesozoic eras.
• Humans: anatomically modern humans (Homo sapiens) evolved within the last few hundred thousand years; current evidence places their emergence at roughly 250 000-300 000 years ago.

The geological timescale

By dating fossils and their surrounding rock strata, scientists have built a geological timescale. The timescale divides Earth history into units: eons (or aeons), eras, periods and epochs. The fossil record and key events are associated with these units.

Using inductive and deductive reasoning in palaeontology

Scientists use both inductive and deductive reasoning to build and test hypotheses about Earth history.

Inductive reasoning

Inductive reasoning uses specific observations to form general ideas or hypotheses. Early ideas about the origin of life were developed by comparing chemical and geological evidence. For example, A. I. Oparin and J. B. S. Haldane proposed in the 1920s that organic molecules could form from simple gases present in a primitive atmosphere under the influence of ultraviolet light and lightning. This idea used observations about chemistry and the early Earth to suggest a pathway for the origin of biological molecules.

Deductive reasoning and laboratory tests

Deductive reasoning uses known facts and laws to infer consequences that can be tested. The famous Miller-Urey experiment (1953) tested the Oparin-Haldane idea by simulating a reducing atmosphere and supplying electrical sparks to represent lightning. Miller's apparatus contained water (H₂O), methane (CH₄), ammonia (NH₃) and hydrogen (H₂). After running the apparatus, Miller collected samples and found a variety of organic molecules, showing that simple chemical building blocks of life can form under plausible early-Earth conditions.

The Cambrian explosion

The Cambrian explosion refers to a relatively brief interval early in the Cambrian period (beginning about 543 million years ago in older conventions) during which many major animal groups appear suddenly and abundantly in the fossil record. In the first few million years of the Cambrian many of the major animal phyla became established.

Major animal groups that first appear or become abundant include:

• Platyhelminthes (flatworms)
• Nematoda (roundworms)
• Annelida (segmented worms)
• Mollusca (snails, clams, cephalopods)
• Arthropoda (insects, crustaceans, trilobites)
• Echinodermata (sea stars, sea urchins)
• Chordata (the group that includes vertebrates)

From Cambrian forms, the fossil record shows the later emergence of major vertebrate and terrestrial groups in this general order: fishes, amphibians, reptiles, birds and mammals.

Mass extinctions

Mass extinctions are intervals in Earth history when a large proportion of species disappeared in a relatively short geological time. There have been five major mass extinctions that greatly reduced global biodiversity:

• End-Ordovician extinction (about 444 million years ago)
• Late Devonian extinction(s) (roughly ~375-359 million years ago)
• End-Permian extinction (about 252 million years ago) - the largest known extinction
• End-Triassic extinction (about 201 million years ago)
• End-Cretaceous extinction (about 66 million years ago) - extinction of non-avian dinosaurs

Causes of mass extinctions

There are multiple hypotheses for the causes of mass extinctions. These fall into two broad categories: catastrophic external events and long-term Earth-bound environmental changes. Often more than one factor was involved.

Earth-bound theories

Ice ages, continental movement, plate tectonics, massive volcanic activity and biotic factors (for example disease) are major Earth-bound causes that can change habitats and climates and lead to extinctions.

Ice ages cause global cooling, growth of large ice sheets, falls in sea level (locking water in ice), lower oxygen levels in some marine environments and changes in salinity. These environmental shifts reduce habitable areas for many species.

Continental drift and plate tectonics change the distribution of land and sea, alter ocean currents and climate, and isolate or connect faunas. The single supercontinent Pangaea existed in the late Paleozoic and early Mesozoic; it later split into Laurasia (northern continents) and Gondwanaland (southern continents). Similar fossils and rock formations found on different continents were important evidence for continental drift.

Volcanism can cause long-lasting climate change. Large-scale volcanic eruptions produce ash and aerosols that reduce sunlight, causing global cooling, and release greenhouse gases (CO₂) that can lead to later warming. Massive volcanic events coincide with several extinction events in Earth history.

Killer diseases and biological invasions can also change ecosystems rapidly; climate shifts may favour pathogens or invasive species that cause widespread mortality.

Extraterrestrial theories

Meteorite and asteroid impacts have affected the climate and environment. A large impact can produce tsunamis, wildfires, acid rain, and a dust and aerosol cloud that blocks sunlight for months or years (a short-term "impact winter"), disrupting photosynthesis and food webs. The impact that formed the Chicxulub crater in the Gulf of Mexico is strongly associated with the end-Cretaceous extinction about 66 million years ago. Large impact structures such as the Vredefort Dome (South Africa) record very ancient collisions.

The present - a sixth mass extinction?

Many scientists warn that current extinction rates are unusually high compared with background rates over geological time. Estimates vary, but some assessments suggest tens of thousands of species may be lost each year if current trends continue. Human activities are the main drivers of this modern biodiversity crisis:

• Habitat loss from deforestation, land conversion for agriculture and urbanisation
• Climate change from increased greenhouse gas emissions
• Pollution of air, water and soil; acid deposition
• Over-exploitation of species by hunting and fishing
• Introduction of invasive species and disease
• Soil degradation through practices like monoculture and over-farming

Fossils and missing links

Fossils are the preserved remains, impressions or traces of ancient organisms. The scientific study of fossils is palaeontology. Fossilisation is the set of processes by which organic materials become preserved in rocks over geological time.

Most fossil preservation occurs in sedimentary rocks. Fossils may be classified by size and type:

• Macrofossils - large body parts or whole organisms visible without a microscope.
• Microfossils - microscopic remains such as pollen, foraminifera or tiny skeletal fragments.

Types of fossil preservation

• Original (unaltered) preservation: rare cases where soft tissues or whole organisms are preserved, for example insects trapped in amber or mammoths frozen in permafrost.
• Petrification / replacement: original tissues are replaced molecule by molecule by minerals (silica, calcite), producing a rock-like replica.
• Permineralisation: mineral-rich water fills pores and cavities in bone or wood and later crystallises, preserving fine structure.
• Moulds and casts: an organism leaves an impression (mould) in sediment; if that mould later fills with sediment that hardens, it forms a cast.
• Compression fossils: remains are flattened and preserved as carbonaceous films or impressions (common for leaves and delicate organisms).
• Trace fossils: indirect evidence of organism activity such as footprints, burrows, coprolites (fossilised dung) or feeding marks.

How fossils form (typical sequence)

1. A plant or animal dies and its remains lie exposed.
2. Sediment (mud, sand, volcanic ash) rapidly covers the remains, protecting them from immediate decay and scavengers.
3. Soft parts decay or are eaten; hard parts (bones, shells) remain.
4. Over time additional layers of sediment bury the remains more deeply.
5. Buried remains are subjected to pressure and chemical action; mineral-laden water percolates through pores.
6. Minerals precipitate and replace or fill the original tissues (permineralisation, replacement) or preserve impressions as moulds and casts.
7. Later uplift and erosion expose the fossil-bearing rocks at the Earth's surface, where palaeontologists may discover them.

The incomplete fossil record and missing links

The fossil record is incomplete because fossilisation is an uncommon outcome: organisms must die in favourable environments (rapid burial, low oxygen, low disturbance) and possess parts that fossilise well (hard skeletons). Soft-bodied organisms are less likely to be preserved unless exceptional conditions occur.

A missing link is an intermediate fossil that shows features bridging two groups. Several well-known transitional fossils are:

• Coelacanth (Latimeria chalumnae): once known only as fossils, a living specimen was caught near the Chalumna River (East London, South Africa) in 1938. The coelacanth shows anatomical traits intermediate between fishes and terrestrial vertebrates and is often called a "living fossil".
• Archaeopteryx: fossils from Solnhofen limestone (Germany) show a creature with both reptilian (teeth, long bony tail) and avian (feathers, wings) features. It is a key link between small carnivorous dinosaurs and birds and lived during the late Jurassic (about 147 million years ago).
• Thrinaxodon: a small mammal-like reptile from the Triassic (about 248-245 million years ago). Its anatomy (for example possible whisker foramina in the skull) suggests transitional characters between reptiles and mammals; fossils found in South Africa and Antarctica support the former connection of those landmasses.

Methods used to date fossils

Two principal methods for assigning ages to fossils are relative dating and radiometric (absolute) dating.

Relative dating

Relative dating determines whether one fossil or rock layer is older or younger than another. In undisturbed sedimentary sequences, lower layers are older than upper layers (the law of superposition). When layers have been disturbed by tectonic activity, geologists use index fossils (fossils that are widespread but restricted to a short interval of geological time) to correlate layers between different sites. Trilobites and ammonites are classical index fossils for parts of the Paleozoic and Mesozoic respectively.

Radiometric dating (absolute dating)

Radiometric dating relies on the predictable decay of radioactive isotopes within minerals. Each radioactive isotope decays to a stable daughter isotope at a known rate, expressed as a half-life (the time for half the original parent isotope to decay).

Common isotopes used and their approximate half-lives:

• Carbon-14 (14C) - half-life ≈ 5 730 years; used for dating recent organic remains (up to ~50 000 years).
• Potassium-40 (40K) - half-life ≈ 1.25 × 10⁹ years; used in dating volcanic and igneous rocks and minerals, and indirectly dating older fossils.
• Uranium-238 (238U) - half-life ≈ 4.468 × 10⁹ years; used to date very old rocks including those several billion years old.

By measuring the ratio of parent to daughter isotopes in a sample and knowing the half-life, scientists calculate the absolute age of the sample and the fossils or strata associated with it.

Southern African evidence for key events

Southern Africa contains many important fossil localities that document major events in the history of life and provide evidence for continental connections such as Gondwanaland.

Soft-bodied animal fossils in Namibia

Ediacaran fossils preserved in sandstone beds in Namibia date to about 600-543 million years ago. These soft-bodied fossils represent some of the earliest complex multicellular life forms. They lack hard skeletons and many palaeontologists interpret some as large lichens or as a distinct group of early multicellular organisms.

Early plants near Grahamstown

Fossils of lycophytes (primitive vascular plants) have been found in the Grahamstown district. Lycophytes show early development of conducting tissues and simple roots and stems, and are important for understanding the origin of land plants.

Permian forests near Mooi River and Estcourt

Permian plant fossils (for example the genus Glossopteris) occur in South African coal fields. Glossopteris had long leaves and produced seeds in cone-like structures; its distribution across South America, Africa, India, Australia and Antarctica provides evidence for the former supercontinent Gondwanaland. Glossopteris forests largely disappeared during the end-Permian mass extinction about 251 million years ago.

Mammal-like reptiles in the Karoo

The Karoo region is rich in Permian and Triassic fossils including early therapsids (mammal-like reptiles), amphibians, fishes and insect remains. Fossils such as Thrinaxodon and Lystrosaurus are well represented. Lystrosaurus, roughly pig-sized with strong forelimbs for digging, is particularly abundant in Karoo strata and in coeval rocks of Antarctica, supporting the connection between southern continents.

The coelacanth in East London

Fossils of coelacanths were known from ancient rocks and were thought to be extinct for some 65 million years. In 1938 a living specimen (a "living fossil") of Latimeria chalumnae was caught near the mouth of the Chalumna River (East London), surprising scientists and showing that rare survivors of ancient lineages can persist in modern oceans.

Dinosaurs and early mammals - Eastern Cape and Lesotho

The Maluti Mountains (Lesotho) and certain sites in the Eastern Cape contain fossils of early dinosaurs and early mammals. Caves such as Melkhoutboom have provided evidence relevant to the origins of early humans.

Dinosaurs in southern Africa

Fossils of large Triassic and Jurassic reptiles have been found across South Africa, Lesotho and Zimbabwe. For example, fossils attributed to genera like Euskelosaurus (a late Triassic/early Jurassic basal sauropodomorph) are known from Drakensberg and Maluti strata.

Early hominids in South Africa

The region known as the Cradle of Humankind (Gauteng) includes the Sterkfontein, Swartkrans and Kromdraai caves and yields many important hominid fossils spanning the last few million years. Notable finds include:

• Mrs Ples - a well-preserved skull of Australopithecus africanus discovered at Sterkfontein and described by Robert Broom (discovery at Sterkfontein in 1947).
• The Taung Child - the skull of a young Australopithecus africanus found at Taung in the North West Province.
• Australopithecus sediba - a more recently described hominid species from Sterkfontein; the discovery (by Matthew Berger and colleagues) was announced in 2010 and the species was given the nickname Karabo.

The fossil record from these sites is central to the Out of Africa model for modern human origins, which proposes that Homo sapiens evolved in Africa and later dispersed to other continents.

Very old bacterial fossils from Barberton

Some of the oldest well-preserved fossil evidence of life comes from the Barberton Greenstone Belt (Mpumalanga), where microfossils and stromatolitic structures indicate simple microbial life over 3.5 billion years ago. These remains document some of the earliest life on Earth.

Fossil tourism and opportunities

Many fossil localities have been developed for education and tourism, providing jobs and economic benefits for local communities while promoting conservation and scientific awareness. Examples include:

Karoo National Park (Western Cape)

The Karoo National Park near Beaufort West has a fossil trail where visitors can see many fossil localities and learn about the geology and palaeontology of the region. The broader Karoo contains fossils of pre-dinosaur tetrapods, early reptiles and therapsids.

Cradle of Humankind (Gauteng)

The World Heritage Site is developed as an educational and visitor attraction with guided tours of fossil sites and cave systems, and exhibits that explain human evolution and the scientific process of discovery.

West Coast Fossil Park (Langebaan, Western Cape)

Fossils discovered at Langebaanweg from the late Pliocene to Pleistocene are exceptionally well preserved and diverse. The site has yielded important finds such as Agriotherium africanum (a fossil bear), and the remains of extinct seals and penguins, and it has been developed as a visitor and research facility.

Summary

The history of life on Earth spans billions of years and is recorded in rocks and fossils. Major transitions include the origin of life, the rise of oxygen in the atmosphere, the evolution of eukaryotes and multicellular organisms, the Cambrian expansion of animal body plans, the colonisation of land by plants and animals, and repeated episodes of diversification and extinction. Scientific methods including inductive inference, hypothesis testing (for example the Miller-Urey experiment), relative stratigraphic correlation and radiometric dating together permit reconstruction of this deep history. Regional fossil records, such as those of southern Africa, supply key examples that illustrate global events and the processes that shape life on Earth.