FOR THE FIRST TWO BILLION OR SO YEARS of life on Earth, the only organisms to exist were single-celled microbes. The evolution of biological complexity was so slow in those early days that scientists have dubbed one vast stretch of this period the ‘boring billion’. Evolution is not for the impatient.
The first animals – multi-cellular life-forms that breathe oxygenmore than a colony of cells arranged so as to be able to filter food from water.