Brave new worlds

The eight novel ways of transmitting information, outlined in the previous chapter as 'major transitions', by no means exhaust evolution's extraordinary feats. Over the history of life, evolutionary events have also radically changed the characteristics of the biosphere. As in the previous chapter then, which considered how evolutionary changes increased the complexity of organisms, this chapter will consider how evolution has increased the complexity of planetary ecology. Four things generally indicate that major ecological changes have occurred in the past, identifiable, for example, from the fossil record (Vermeij 1995; Kanygin 2001): First, changes in species richness. Second, organisms living in new places. Third, ecosystems with new functional groups. Fourth, new geochemical cycles. For present purposes we are only interested in such changes that are linked with evolutionary events, as most (but not all) of them are by definition. The changes need collectively to create the essential and complex features of modern planetary ecology, which are worth briefly describing.

Species richness is currently (recent extinctions excepted) higher than ever before (Signor 1990; Sepkoski 1999). About half of described macroscopic species are insects, a quarter green plants, and most of the rest sundry invertebrates (Southwood 1978). Life exists and flourishes nearly everywhere on the face of the globe; in the marine, freshwater and terrestrial realms, anoxic mud, animal guts, hypersaline lakes, volcanic springs, desert dunes, within frozen antarctic rock and in rocks hundreds of metres below ground. Some organisms even live an essentially atmospheric existence, feeding on other flying organisms.

The most productive and diverse ecosystems on the planet are terrestrial. They comprise: green plant producers; a diverse assemblage of vertebrate and insect herbivores; predators, parasites and parasitoids, and a soil fauna of scavengers, detritivores, decomposers, and nutrient cycling bacteria. In the marine realm the most productive and species-rich ecosystems are those with sessile producers, such as coral reefs, kelp forests, and seagrass beds. The open ocean is a comparative desert, but consists of pelagic phytoplankton, zooplankton, and macroscopic predators. A benthic fauna consists of filter feeders, scavengers, and predators. The biosphere's energy comes, almost exclusively, from light captured by plants, and its carbon source is atmospheric

CENOZOIC CRETACEOUS JURASSIC

TRIASSIC PERMIAN

CARBONIFEROUS

O DEVONIAN Z

J SILURIAN ORDOVICIAN CAMBRIAN

PROTEROZOIC Oxic world

ARCHEAN Life begins

HADEAN Earth forms

Ma 0

- 417 440 495 535

-2500

-3500

4500

Possible date

Novelty

Why a 'major transition'?

(Ma)

■ 140

Flowers

New trophic relationships, increase in species richness

■ 350-50

Flight

Spatial expansion of species, increase in species richness

350

Phytophagy

New trophic niches, increase in species richness, changes to biogeochemical cycles

350

Trees

Increase in species richness, changes to biogeochemical cycles

430-390

Terrestrialization

Spatial expansion of species.

535

Major animal

Increase in species richness, new

lineages

trophic niches, change to biogeochemical cycles,

3500-2200

Phagotrophy

New trophic niche

■ 3500-2500

Aerobic respiration

Change to biogeochemical cycles

3500

Oxygenic photosynthesis

Change to biogeochemical cycles

3500

Photosynthesis

Change to biogeochemical cycles

3500

Heterotrophy

New trophic niche

Fig. 3.1 Some major transitions in ecology resulting from evolutionary novelty, and when they occurred.

carbon dioxide, which is eventually returned by (high energy-yielding) aerobic respiration. Where, then, did all this come from and what evolutionary novelties helped it get there? Below I tentatively identify eleven major changes that take us from the origin of life to an ecologically modern planet (Figure 3.1).

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