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Adding time to the tree
If you wanted to squeeze the 3.5 billion years of the history of life on Earth into a single
minute, you would have to wait about 50 seconds for multicellular life to evolve, another four
seconds for vertebrates to invade the land, and another four seconds for flowers to evolve — and
only in the last 0.002 seconds would "modern" humans arise.
Biologists often represent time on phylogenies by drawing the branch lengths in proportion
to the amount of time that has passed since that lineage arose. If the tree of life were
drawn in this way, it would have a very long trunk indeed before it reached the first plant
and animal branches.
The following phylogeny represents vertebrate evolution — just a small clade on the tree of
life. The lengths of the branches have been adjusted to show when lineages split and went extinct.
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