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The unity of the genetic code, the DNA instructions that specify the make-up of the proteins that the bodies of organisms are built from, confirms that all life on earth had a common ancestry.
Charles Darwin sketched the branches of a tree in one of his notebooks to illustrate the idea that all life forms branched out from a common ancestry. Since then the tree of life image has persisted as an aid to understanding evolution.
However, the analogy has its drawbacks. It reflects an outdated assumption that evolution involves progression from 'primitive' to 'developed', from 'simple' to 'complex' and hence from 'inferior' to 'superior', with the human race at the top. Modern evolutionary theory carefully excludes such value judgements and views evolution as more like tangled undergrowth than a branching tree.
If the tree idea is inadequate, so is our diagram. There are way too many life forms for us to feature them. The sample shown is highly biased towards those that can still be found alive; the majority are now extinct. It favours organisms that humans generally know and like, giving less attention to those we dislike or don't often come across. Insects, for example, are grossly under-represented, as are parasitic groups which, though abundant in reality, hardly appear here at all. There are only a few examples of microbes near the foot of the tree whereas in fact they form a vast and complex interrelated network of organisms.
So there are many reasons to treat this exercise with caution. But it remains an aid to understanding and we hope it will help you appreciate both the diversity and the interrelatedness of living organisms.
References: The Open Univesity - Tree of Life