The Ordovician period was a time when life flourished. But towards its end, the world cooled dramatically and ice sheets spread from the poles. The ensuing ice age is called the Andean-Saharan, because the evidence of it comes from the Andes mountains and the Sahara desert. The deep freeze led to the second-worst mass extinction on record, the Ordovician-Silurian. Most life was still confined to the sea, and 85% of marine species were wiped out. In the aftermath, fish became much more common.
Any of a group of minerals characterized by highly perfect cleavage, the tendency of crystalline materials to split along definite crystallographic structural planes, which explains why it readily separates into very thin leaves. It is used in electric conductors, roof insulation, house paint, soil conditioner, cosmetics and toothpaste.
‘Wholly, holly, signal, signing,
Same, examining, but mining,
Scholar, vicar, and cigar,
Solar, mica, war and far.’
– Gerard Nolst Trenité, The Chaos
Soon after animals evolved, evolution went through two major growth spurts. In the Cambrian Explosion, it seems almost every group of modern animals appeared within tens of millions of years. This apparent ‘explosion’ may be partly down to better fossilisation, as many animals now had hard shells. Then 489 million years ago, each animal group expanded in the Great Ordovician Biodiversification Event.
Earth froze over again, twice, in the space of 200 million years. The ice may well have stretched all the way from the poles to the equator. This second Snowball period may have triggered the evolution of the first complex animals. The first complex organisms, weird tube- and frond-shaped things called the Ediacarans, appeared soon after.
For the first time, life was not just made up of single cells. Now cells were teaming up to form larger organisms with things like mouths, limbs and sense organs. It’s hard to say when this happened: there are fossils of large organisms dating back 2.1 billion years, but these may simply have been colonies of bacteria. Different groups of organisms probably evolved multicellularity independently, with plants managing it before animals.
Between 1.8 billion and 800 million years ago, the fossil record looks fairly dull – so much so that the period is called the ‘Boring Billion’. But behind the scenes plenty was happening. For one thing sex may have evolved for the first time. It’s not clear why, or when, some organisms stopped simply dividing in two and started the messy business of sex. But it was definitely going on 1.2 billion years ago: there are fossils of red algae from that time that were clearly forming specialised sex cells such as spores.
The first organisms were simple cells like modern bacteria, but some of them became much more internally complex. These ‘eukaryotes’ developed lots of specialised equipment within their cells. They also had a new source of energy: sausage-shaped objects called mitochondria that were once free-living bacteria, but which were absorbed in a process called endosymbiosis. Every animal and plant you have ever seen is a eukaryote.