Evolution of Life (Leaving Cert CASD): Revision Notes

Evolution of Life

Early life on Earth

The beginning - an oxygen-free world

In Earth's early history, our planet looked very different from today. The environment was harsh and unwelcoming to most life forms we know now. There was no oxygen in the atmosphere, and methane levels were high. This meant that for much of Earth's history, it would not have supported the complex animals we see today.

The earliest evidence of life comes from microscopic organisms called microbes that lived approximately 3.7 billion years ago. These tiny life forms left behind chemical signatures in ancient rocks - special carbon molecules that only living things can produce. Unlike modern animals that need oxygen to survive, these early microbes lived without it entirely.

Evidence from stromatolites

Scientists have found additional proof of early life in structures called stromatolites, dating back about 3.5 billion years ago. Stromatolites are hard, layered structures formed when sticky mats of microbes trap and bind sediments together. As minerals settle within these layers, they create lasting structures that survive even after the microbes die.

Today, researchers study rare living stromatolite reefs to better understand how Earth's earliest life forms operated and survived in such different conditions.

Development of an oxygen atmosphere

A revolutionary change occurred around 2.4 billion years ago when cyanobacteria evolved. These remarkable organisms became Earth's first photo-synthesisers, using water and the Sun's energy to make food. As a by-product of this process, they released oxygen into the atmosphere.

Evidence for this massive environmental change can be found in Banded Iron Formations (BIFs) - special seafloor rocks with distinctive dark, iron-rich layers. When oxygen-rich shallow water mixed with deep, iron-rich water, chemical reactions occurred. The iron combined with oxygen to form iron oxide minerals, which sank to the seafloor and created these characteristic layered rocks.

After this initial oxygen surge, atmospheric levels stabilised at lower concentrations for about two billion years more. The ocean remained unsuitable for most oxygen-dependent life forms during this time.

Rise of multicellular life

While these major atmospheric changes were happening, other important innovations were taking place. Although microbes could process many chemicals, they lacked the specialised structures needed for complex body plans.

A groundbreaking development occurred when microbes began living inside other microbes, functioning as internal components called organelles. Mitochondria - the structures that convert food into energy - evolved from these cooperative relationships between different microbes.

For the first time, DNA became packaged inside protective structures called nuclei. These new complex cells, known as eukaryotic cells, contained specialised parts that each performed specific functions to support the entire cell.

Some cells created connections to hold the group together, while others produced digestive enzymes to break down food. This cellular specialisation became the foundation for all complex multicellular life.

The first animals

Sponges and early body plans

These clusters of specialised, cooperating cells eventually became the first animals around 800 million years ago. Sponges were among the earliest animal forms. Even though ocean oxygen levels remained low compared to today, sponges could tolerate these conditions because they weren't very active animals and didn't require large amounts of oxygen for their metabolism.

Sponges have a simple body structure consisting of layers of cells surrounding water-filled spaces, supported by hard skeletal components. They feed by extracting food particles from water pumped through their bodies by specialised cells. The development of increasingly complex and diverse body plans would eventually lead to distinct animal groups.

Thanks to their hard skeletons, sponges became Earth's first reef builders. Thousands of sponge species continue to thrive on our planet today.

Ediacaran period organisms

Around 580 million years ago (during the Ediacaran Period), there was a remarkable increase in the variety of organisms beyond sponges. These fascinating seafloor creatures had unusual body shapes - some looked like fronds, ribbons, or even quilts. They lived alongside sponges for about 80 million years, and their fossil remains can be found in sedimentary rocks worldwide.

However, most Ediacaran animals had body structures unlike any modern animal groups. Scientists continue to investigate whether any of these fossilised Ediacaran creatures were related to animals we know today.

By the end of the Ediacaran period, oxygen levels had risen significantly, approaching concentrations sufficient to support oxygen-dependent life. The early sponges may have actually helped increase oxygen levels by consuming bacteria and removing them from the natural decomposition process.

Evidence suggests that some organisms, like Dickinsonia costata, moved along the sea bottom, probably feeding on mats of microbes.

The end-Ediacaran extinction

However, approximately 541 million years ago, most Ediacaran creatures disappeared in what scientists call a major environmental change. Researchers are still working to understand the causes of this extinction event, but it likely involved changing body plans, feeding relationships, and environmental engineering.

The Cambrian explosion

Rapid diversification of life

The Cambrian Period (541-485 million years ago) witnessed an extraordinary explosion of new life forms. Along with new burrowing lifestyles came hard body parts like shells and spines. These hard structures allowed animals to engineer their environments more dramatically, such as by digging elaborate burrows.

A significant shift also occurred towards more active animals with defined heads and tails for directional movement and chasing prey. Active feeding by well-armoured animals like trilobites may have further disrupted the seafloor where soft Ediacaran creatures had previously lived.

Unique feeding strategies and diversification

Different feeding styles divided up the available environment, creating opportunities for greater life diversification. In 1909, scientists discovered the Burgess Shale fossils, which revealed the extraordinary biodiversity of Cambrian life.

Some groups, like trilobites, thrived and dominated Earth for hundreds of millions of years but eventually became extinct. Stromatolite reef-building bacteria declined, and reefs built by organisms called brachiopods developed as Earth's conditions continued changing. Today's dominant reef-builders - hard corals - didn't appear until several hundred million years later.

Establishment of modern animal foundations

Despite all the changes that would continue, by the end of the Cambrian period, nearly all existing animal types or phyla (including molluscs, arthropods, and annelids) had been established. Complex food webs were developing, creating the foundations for today's ecosystems.

This period established the basic framework for life on Earth that continues today, even though many specific groups have come and gone through subsequent evolutionary history.