How Life's Structures and Functions Changed Over Time

A 4.6-billion-year story

In this unit you have met prokaryotes, protists, fungi, plants and animals. They did not all appear at once. The fossil record and DNA show a long story in which simple cells gave rise, step by step, to more complex structures. Each new structure did a new function, and opened up new ways of life.

This lesson follows the biggest of these changes. You will study how evolution works (natural selection, speciation) in the Evolution unit; here the focus is on what changed.

1. The first cells were prokaryotes

Earth formed about 4600 million years ago (Mya). Some of the oldest widely accepted evidence of life is about 3500 Mya: fossil stromatolites, layered mounds built by mats of microbes. For well over a billion years, all life was prokaryotic and unicellular.

Around 2400 Mya, cyanobacteria that performed oxygen-releasing photosynthesis caused oxygen to build up in the air: the Great Oxidation Event. Oxygen was poisonous to many microbes, but it also allowed aerobic respiration, which releases far more energy from food than fermentation does. It also built the ozone layer that later protected life on land from ultraviolet light.

2. Eukaryotic cells: the endosymbiotic theory

Eukaryotic cells appear in the fossil record roughly 2000 Mya (estimates vary). Where did their mitochondria and chloroplasts come from? The endosymbiotic theory, championed by American biologist Lynn Margulis in the 1960s, says:

1. A large host cell (which may already have had a nucleus formed from folds of its membrane) engulfed an aerobic bacterium but did not digest it.
2. The bacterium lived inside, supplying energy; the host supplied protection and food. Over time it became the mitochondrion.
3. Later, one of these cells engulfed a cyanobacterium, which became the chloroplast. Its descendants are algae and plants.

Evidence:

• Mitochondria and chloroplasts have their own circular DNA, like bacteria.
• They have their own 70S-type ribosomes, like bacteria.
• They have two membranes, as expected if a cell was wrapped in the host's membrane.
• They divide by a process like binary fission, and cells cannot make new ones from scratch.
• They are about the size of bacteria, and their DNA sequences are most similar to particular bacteria (mitochondria to a group of aerobic bacteria; chloroplasts to cyanobacteria).

3. Multicellularity and sexual reproduction

• Multicellularity evolved several separate times (in algae, fungi, plants and animals). By about 1000 Mya there were multicellular algae. Being multicellular allows cell specialization: some cells handle feeding, others protection or reproduction. Organisms can grow larger, live longer and escape small predators. Colonies such as the green alga Volvox, where some cells specialize in reproduction, show what an early step might have looked like.
• Sexual reproduction (with meiosis and fertilization) combines genes from two parents. Offspring are genetically varied, so populations can adapt faster when conditions change. You will study meiosis in the Genetic Processes unit.

4. Animal body plans: Canada's famous fossils

Some of the oldest known large, complex multicellular organisms (many of them probably early animals) are found at Mistaken Point, Newfoundland and Labrador, a UNESCO World Heritage Site, where soft-bodied Ediacaran organisms are preserved in rock about 565 million years old.

Beginning about 538 Mya, in the Cambrian explosion, most animal phyla appeared in the fossil record within a geologically short time. The Burgess Shale in Yoho National Park, British Columbia (about 508 million years old), discovered by Charles Walcott in 1909, preserves even soft-bodied Cambrian animals in amazing detail. Palaeontologists at the Royal Ontario Museum and other institutions are still describing new species from it.

Cambrian animals show key structural innovations: hard parts (shells, exoskeletons), eyes, bilateral symmetry with a head, segmentation, a complete gut, and early chordates with a notochord.

5. Moving onto land

Plants (about 470 Mya, descended from green algae) evolved a waxy cuticle and stomata to prevent drying out, vascular tissue with lignin to carry water and stand tall, then seeds and pollen to reproduce without swimming sperm, and finally flowers and fruit (about 130 Mya).

Animals followed plants. Arthropods were among the first. For vertebrates, a key fossil is Tiktaalik roseae, a transitional fossil about 375 million years old, found in 2004 on Ellesmere Island, Nunavut. It had gills and scales like a fish, but also lungs, a flat head with a neck, and sturdy fins with wrist-like bones. Its name was chosen with Inuit elders of Nunavut; tiktaalik is the Inuktitut word for a large freshwater fish (the burbot). Later came four limbs, then the amniotic egg (about 312 Mya), which let reptiles, birds and mammals reproduce away from water.