When the Earth Was Green

A fascinating book that the traces the evolution of plants and the relationship plants have with animals, via a series of vignettes that look at the life of plants and animals at different periods of time. An appendix of information is also given to provide the scientific background to the vignettes, followed by a list of references.

While fossils of animals (especially dinosaurs) fascinate the public and are the usual 'stars' of palaeontology, plants are the ones that fuel those bodies directly or indirectly. Without plants, there would be no animals, and plants determine what kinds of animals can exist in areas of the world. So it is worthwhile to get an understanding of how plants evolved to understand more of the world that prehistoric animals inhabit.

Our first stop is at the beginning, when the first plants appeared. At some point in time, single-celled organisms in an ocean swallowed cyanobacteria, leading to the first multicellular organisms that could produce food from sunlight. These organisms become more than just a group of cells by differentiating and taking on different roles that benefit the organism as a hold; like cells that hold on fast on to rocks, and helping the organism to reach higher in the water for sunlight. Sex would also arise from differentiation, leading to more variations in subsequent organisms.

Plants now begin to line the shoreline and their growing numbers begin to affect the environment. One of the biggest of these would be the creation of free oxygen molecules, and freeing minerals like calcium from the rocks. They also begin to suck carbon dioxide from the air, resulting in a cooling climate.

Moving on to the Carboniferous, plants now dominate the landscape. The plants now found are not the plants we know, but consist mainly of giant liverworts and ferns that tower metres into the air. Plants achieve this ability to grow tall by repurposing lignin for support (lignin may have originally evolved to protect plants from ultraviolet rays). These plants support an increasingly complex ecosystem made up of giant insects and proto mammals and reptiles, supported by the excess oxygen created by plants. Insects, especially those with an aquatic life cycle, grew big as a response to the extra oxygen, which can be toxic at higher concentrations: a larger body is better able to handle the excess oxygen. Plants grew so rapidly that there wasn't time for dead plants to be decomposed before getting buried, leading to the large coal seams that we find today.

Next, we go to the Triassic, a period of extremes due to the volcanic activity, causing high levels of carbon dioxide and methane in the atmosphere. The climate now has hot seasons and wet ones, causing wildfires and deluges that bury the burnt wood; when fossilised, this wood would become petrified forests found in some areas.

We move to the Jurassic, the era of giant sauropod dinosaurs that need to feed on many plants to fuel their bodies, like ferns, conifers and ginkgos. Sauropods start out small, and they need to grow quickly to avoid being targeted by predators. Their huge bodies also help them digest plants, for sauropods cannot chew and just swallow the plants they eat, along with unlucky small animals.

Now in the Cretaceous period, a new type of plant develops: angiosperms, or flowering plants. They probably grew mainly in watery areas due to competition from ferns and other gymnosperms that are already established.

The Cretaceous period is also one where plants have spread widely, nearly reaching the poles. Large numbers of conifer trees produce resin, which turn into amber when fossilised. These preserve insects or parts of animals that have been trapped within them.

A large asteroid strike that ends the reign of the dinosaurs moves us past the Cretaceous. The destruction does not spare plants. But it gives the angiosperms a chance to spread, and they eventually dominate in the new landscape. The climate after the asteroid strike remains hot, giving rise to large storm that can sweep whole groups of trees out into the oceans. Some of these 'rafts' include stowaways, and this was probably how monkeys made the leap across the ocean from Africa to South America.

The climate now starts to cool, bringing with it seasons and a reduction in rainfall. Forests starts to retreat, opening the ground for grasses and a new environment for animals. Forests gives cover for predators to hide in ambush, but on grassy plains with no cover, predators have to adapt to chase after their prey. One way prey animals can respond is by becoming faster, like horses, which move to grassy plains to take advantage of the new environment.

The author now looks at the relationship between plants and animals by looking at plant pollinators like bats. Some plants also evolved from being general pollination to specialised one, developing plant parts that only some animals can reach as a reward for pollination. This ensures that plants have a higher change of receiving the correct pollen. But if the specialised pollinator goes extinct, the plant is also in danger of becoming extinct, unless it can find a new pollinator to fit the role.

The interesting effects of catnip on cats is also looked at, with a look at a possible behaviour of prehistoric sabre-tooth cats to catnip. Catnip plants developed a chemical to deter animals from eating it. But cats instead find the chemical attractive.

Moving back to grass plains, it also attracts another animal: primates. Some come down from the trees to move on the ground, develop an upright stance. And some would go on to become humans.

Finally, nearing modern times, the author looks at mastodons and mammoths. The changing seasons would also cause season, with fall causing a shedding of leaves. This blanket of fallen leaves would be an aid for small animals to survive the winter, by providing insulation on the ground. A look at parasitic plants that live off fungi and other plants is also given.