Imagine a world whose land surfaces were totally barren, and whose organisms were so reduced in number that life had to recolonise those surfaces from scratch, from isolated remnants that had somehow survived cataclysmic destruction. In what order would organisms have begun to appear in the recovery process?

Amongst the most important factors affecting that order would be: reproduction rate, ability to disperse, the nature of the land surface (rock, clay or sand), and climate. Another important factor would be ecological succession. Organisms do not live in a vacuum: they live in ecological communities and participate in food chains or food webs where larger organisms depend on smaller ones, and the smallest organisms depend on inorganic sources of energy.

Bacteria should not be seen as mere leftovers from an evolutionary process that soon moved on to higher things. Whether in the soil or in the human gut, they play a vital role, and more complex organisms are completely dependent on them. The same is true of other relatively simple (in absolute terms, far from simple) organisms such as lichens and liverworts. They help to make the world habitable for other organisms.

Thus, as an ecological community develops, organisms at the bottom of the food chain establish themselves before those at the top. Rates of reproduction tend to decrease in line with this sequence. Bacteria reproduce prolifically, whereas trees produce fewer offspring and take longer to reach maturity. An ecological community has a pyramid-like structure, with the organisms on which all the others depend being the smallest and most numerous and organisms further up the food chain being progressively larger and less numerous. The pyramidal structure reflects the fact that the higher organisms consume more energy, which has to come from those further down the food chain.

Some of the best places for illustrating ecological succession are the granite outcrops of Western Australia. Dating back to the geological era known as the Archaean, the granites are extremely ancient and in some places have been subject to chemical and physical weathering ever since their formation. They formed rapidly, with only brief intervals between one eruption and the next, so they do not contain fossil soils. At the top we see a colonisation process, as rock is laid bare.

	1. General view of Boyagin Rock, on the Darling Plateau (Yilgarn craton).
2. Lichens penetrate the granite with filaments (hyphae), break up the minerals into smaller crystal grains and produce weak acids, so that the surface begins to form a shallow soil. The pink patches are bare oxidised rock, exposed as whole layers have come away.
	3. Acidified by the lichens and chemicals from the soils, rainwater accelerates erosion. Wind and temperature fluctuations also play their part, clearing away entire layers of granite and forming depressions where the cycle begins again. In the distance grasses begin to take hold.
4. Increased soil depth on the rock allows shrubs such as Borya constricta, the prickly Pincushion Plant, to grow.
	5. The whole succession from lichens to mature trees. The trees take root downslope, where granite-derived soils have accumulated to a considerable depth.

Such successions help us to understand the fossils contained in the stack of successive land surfaces that make up the geological record. If it is true that rock strata accumulated as the world recovered from catastrophe and became progressively more habitable, the order in which fossils appear should reflect a process of ecological recovery.

The succession will not of course be the same everywhere. A dry desert, for example, is colonised in a different way from a delta plain. In general, nonetheless, the first organisms to make their mark on the fossil record would be bacteria and algae, after them fungi, lichens and liverworts (none of which fossilise well), then small plants, then larger plants. Once the environment was sufficiently prepared, around the small plants stage, invertebrate animals (such as millipedes and insects) would start to play a role in developing the environment.

The competing theory is that the sequence from bacteria to algae (in marine environments) to lichens, liverworts and mosses, to small plants, then larger plants, is an evolutionary sequence. Can you think of criteria which would enable us to decide between the two interpretations?