10.3 Life's History

Pre-Cambrian (ESGCM)

Hadean: the Earth we live on is approximately \(\text{4,6}\) billion years old. The conditions that allowed for the emergence of life lasted approximately \(\text{500}\) million years. It was an environment in which the Earth's crust cooled and the oceans and atmosphere began to form. In this environment, a variety of complex chemical reactions occurred, resulting in the production of the earliest molecules capable of making copies of themselves.

Archean: when life in the form of uni-cellular organisms first developed, the Earth's early atmosphere consisted entirely of volcanic gases, and there was no free oxygen. Prokaryotes evolved, with photosynthesising bacteria (known as cyanobacteria) emerging approximately 3 billion years ago. The presence of photosynthesising cyanobacteria resulted in the release of oxygen into the atmosphere.

The early single-celled organisms (bacteria and cyanobacteria) lived together in aquatic colonies. These colonies were formed by trapping sediments and minerals floating in water and by producing a mucus which bound everything together. As the colony of bacteria, minerals and sediments grew, so did the structure that they made, and layer upon layer built up. In order to capture as much sunlight as possible, the shape of the top of this sedimented algal mat was curved. These curved and multi-layered structures, called stromatolites, were preserved and exist as fossils. Some stromatolites survive to this day, in scattered locations around the globe (Figure 10.6).

Proterozoic: over the next 800 million years, the earliest forms of sexual reproduction developed, thus greatly increasing the diversity of organisms. The first multicellular organisms consisting of cells specialised to perform specific functions began to evolve. The soft-bodied organisms, known as Swartpuntia, first appeared approximately \(\text{600}\) million years ago and went extinct by the start of the Cambrian, \(\text{543}\) million years ago. Swartpuntia fossils were discovered in Namibia and are some of the oldest fossils known to exist from before the Cambrian period.

Cambrian explosion (ESGCN)

From approximately \(\text{580}\)–\(\text{500}\) million years ago, a large diversity of creatures appeared. This event is called the Cambrian Explosion, and almost all of the animals alive today can trace their beginnings to this rapid expansion of diversity. The Cambrian Explosion was the relatively rapid appearance over millions of years of most of the main animal groups found in the fossil record.

In Cambrian times there was no life on land and little or none in freshwater — the sea was still very much the centre of living activity. From \(\text{580}\)–\(\text{540}\) million years ago, oxygen began to accumulate in the atmosphere, allowing for the formation of the ozone layer which blocked the damaging rays of the ultraviolet light from the sun, permitting the colonisation of land.

The Cambrian explosion was just the first period of the Paleozoic era. It is famous because of the sudden explosion in diverse morphologies, and the fact that is "sowed the seeds" for the continued evolution that occurred throughout the Paleozoic.

Paleozoic era (542 to 251 million years ago) (ESGCP)

The massive supercontinent Pangaea formed during the Paleozoic, and this era also saw the diversification and evolution of many of the animal phyla that are still present today. During the Paleozoic:

• The first fish (chordates) appeared more than \(\text{500}\) million years ago, and it is from this common ancestor that vertebrates, including mammals (and humans) are ultimately descended. Fish developed bony spines and jaws with teeth which increased the size and diversity of the prey they could capture.
• Animals with shells and exoskeletons (e.g the trilobites) were in abundance early in the Paleozoic, but declined towards the end of the era.
• The earliest plants colonised land approximately \(\text{430}\) million years ago, having evolved from the nearby algae growing along lakes and other coastal areas.
• Forests of primitive plants covered the Earth's surface, resulting in increased oxygen levels and decreased carbon dioxide levels in the earth's atmosphere.
• The first insects appeared, and later on some species developed wings.
• Later in the Paleozoic amphibians became common and diverse and started moving onto land.
• Early reptiles (synapsids) colonised land.
• Towards the end of the Paleozoic gymnosperms (seed-bearing plants) replaced much of the earlier plant-types.

The end of this period was marked by a mass extinction event (called the Permian-Triassic extinction event) which eliminated almost all marine life but had a less devastating impact on terrestrial species.

South African fossil record: fossils of early land plants such as clubmosses, lycopods, ancestral gymnosperms and algae were found near Grahamstown and Port Alfred. Fossils of plants belonging to the Glossopteris flora are found in the Karoo. This plant flourished throughout the ancient landmass of Gondwanaland. However, by the end of the Permian period this crop had become extinct. The fossilised remains of these plants are found in silts and in rocks throughout the country, where coal deposits can be found (Figure 10.9). Among the organisms that moved on to land at this time were the ammonoids, which are marine invertebrate animals with spiral-shaped shells, the fossils of which are found in the Makhatini flats in KwaZulu Natal.

"Living fossils" in South Africa: Cycads have been in existence for over 280 million years. There are several rare species of cycads found exclusively within South Africa. They grow very slowly and live for up to a 1000 years. Because they are such a rare species, there are several restrictions on what can be done to them. In South Africa, unless required for conservation purposes, it is illegal to collect, pluck, destroy, export or possess cycads without a special permit.

Mesozoic (251-72 million years ago) (ESGCQ)

The Mesozoic Era is often referred to as the 'age of reptiles' because throughout the marine and terrestrial habitats, reptiles (especially dinosaurs) were dominant. This era is divided into three major periods: Triassic, Jurassic and Cretaceous.

The major events of this era included significant changes in the climate, evolutionary activity and land mass of Earth.

• The climate of the Mesozoic fluctuated dramatically between cooling and warming periods.
• Until this point, the Earth existed as one giant land mass called Pangaea. During the Mesozoic era, this land mass broke up, and by the end of the era, the continents as we know them today drifted into their current positions.
• The extinction of nearly all animal species in the former Paleozoic era led to rapid evolution of many new life forms.
• Dinosaurs appeared on land, becoming the major terrestrial vertebrates for nearly 135 million years.
• Pterosaurs (flying reptiles) dominated the skies and aquatic reptiles such as ichthyosaurs were found in the oceans.
• During the Triassic, Archaeopteryx, a link between reptiles and birds was thought to evolve.
• The earliest birds appeared during the Jurassic period having evolved from an order of dinosaur called theropods.
• Dinosaurs became extinct around 65 million years ago, at the end of the Cretaceous period.
• The dominant land plant species of the time were the seed-producing plants known as the gymnosperms. These include the cycads and conifers.

The existing sequoia trees found mainly in the United States were also thought to have evolved in the Mesozoic.

South African fossil examples: The mammal-like reptile, Lystrosaurus was by far the most dominant terrestrial vertebrate during the Early Triassic period. Specimens of the Lystrosaurus were unearthed in the Balfour and Katburg formations in the Karoo. The discovery of Lystrosaurus in the Coalsack Bluff in the Transantarctic Mountains helped confirm the theory that the continents of the Earth were once joined, as Lystrosaurus had already been found in the Early Triassic fossil record of Southern Africa, India and China.

"Living fossils" in South Africa: Coelacanths were thought to have gone extinct towards the end of the Cretaceous period but were re-discovered in 1938 off the northern coast of South Africa in KwaZulu-Natal. The coelacanth is thought to have evolved into its current form over 400 million years ago and is nicknamed a 'living fossil' because knowledge of the species was previously based on fossils, as it was thought to have gone extinct. Coelacanths and amphibians share a common ancestor- the lobe-finned fish. Lobe-finned fish have fleshy fins unlike fins in other fish which are joined to the body by a single bone. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. These fins evolved into legs in the earliest tetrapod land vertebrates which were amphibians. Living lobe-finned organisms include the coelacanths and lungfish.

Cenozoic era (65 million years ago to present) (ESGCR)

If the Mesozoic was the "age of dinosaurs", then the Cenozoic has been the "age of mammals". The Cretaceous-Paleogene extinction event occurred towards the end of the Mesozoic era and half the animal species on Earth, including the dinosaurs were wiped out. As a result mammals diversified to fill ecological niches.

• The continents moved into their existing locations, with Australia-New Guinea which split from Gondwana during early Cretaceous, drifting north and eventually colliding with Southeast Asia. Antarctica moved into its current position over the South Pole and the Atlantic Ocean widened and South America became attached to North America.
• \(\text{54}\)–\(\text{33}\) million years ago, Africa was an island, not yet joined to the Middle East and Europe.
• The climate warmed significantly until 55 million years ago, followed by long-term cooling since 49 million years ago.
• Many species of grass evolved from flowering plants between 33 and 24 million years ago.
• As the Earth cooled, grasslands began to expand and forests shrank.
• Animals evolved to fit the new, open landscape and many fast-running prey and predator species arose as a result.

It is during the Cenozoic Era that our species, Homo sapiens evolved into the anatomically modern humans we are today.

The evolution of Homo sapiens: evidence from Africa(ESGCS)

Humans are thought to have evolved from their ape-like ancestor over the last 6 million years. Human evolution has involved changes in physical appearance and behavioural traits. Some important physical developments include bipedalism, and a larger, more complex brain. Bipedalism is the ability to stand upright on two legs; it was an important adaptation as it freed the hands to be used in other tasks. Developments in behavioural characteristics include the ability to use fire, the ability to make and use tools, and the ability to communicate by means of language.

The most widely accepted theory of how anatomically modern humans evolved, suggests that Homo sapiens evolved in Africa approximately \(\text{200 000}\) to \(\text{150 000}\) years ago in what has now become known as the "Out of Africa" theory. Some of the evidence for the key events as found in Africa is shown in the table below.

Human ancestors evolved and migrated out of Africa in a series of 'waves'.

Fossil sites from South Africa providing evidence for pre-humans and humans

South Africa has been the site of various fossil discoveries and has been home to some of the world's leading paleoanthropologists such as Robert Broom, Raymond Dart and Phillip Tobias. Fossils of pre-humans and humans found across the country have contributed significantly to our understanding of human history on Earth. The table below shows some of the evidence and where it is located in South Africa.

Philip Tobias (October 1925–June 2012) was a South African palaeoanthropologist at the University of Witwatersrand, whose work has contributed enormously to our understanding of how humans evolved. He is best known for his work with fellow palaeoanthropologist Louis Leakey. He identified the ancestral species of human beings known as Homo habilis. His work at the Sterkfontein caves in Gauteng has led to it becoming a World Heritage site. He was also famous for his opposition to apartheid and gave speeches condemning apartheid at protest rallies and also to academic audiences.