SIGNIFICANT STAGES IN THE EVOLUTION OF ANIMALS
Draw an arrow on the tree of life to indicate at which point the earliest animals first appeared.
0 10 20 30 40 50 60 70 80 90 100
Time since life appeared on Earth (expressed as a percentage of total)
Describe what life on Earth was like in the period of time shown as X on the diagram. Mainly unicellular, or simple (not truly multicellular).
One species appears to have been in existence since the very earliest times. Identify that species and describe how it features in the lives of modern mammals including humans. Escherichia coli (E. coli) are bacteria found in huge numbers in almost everyone’s faeces (and those of other mammals as long as they are not hibernating – E. coli like to be warm), where they help break down the solid waste and promote the uptake of vitamin K and other important nutrients. These bacteria cause disease only if they escape from the gut and proliferate in the blood or elsewhere, or if those normally found in other species are eaten with contaminated food. Most other kinds of bacteria are not pathogens, but break down dead and dying plants and animals and fungi.
The earliest multicellular animal phylum members are not considered to be direct ancestors of the more complex animals. Identify this phylum within the Tree of Life and state why this is so.