The common incest tree
Universal common ancestry (UCA) is a central pillar of modern evolutionary theory. As first suggested by [Charles] Darwin, the theory of UCA posits that all extant terrestrial organisms share a common genetic heritage, each being the genealogical descendant of a single species from the distant past. The classic evidence for UCA, although massive, is largely restricted to 'local' common ancestry—for example, of specific phyla rather than the entirety of life—and has yet to fully integrate the recent advances from modern phylogenetics and probability theory. Although UCA is widely assumed, it has rarely been subjected to formal quantitative testing and this has led to critical commentary emphasizing the intrinsic technical difficulties in empirically evaluating a theory of such broad scope1. Furthermore, several researchers have proposed that early life was characterized by rampant horizontal gene transfer, leading some to question the monophyly of life.
The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. ... Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype. But analogy may be a deceitful guide. Nevertheless all living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure, and their laws of growth and reproduction.
A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities—their phylogeny—based upon similarities and differences in their physical or genetic characteristics. The taxa joined together in the tree are implied to have descended from a common ancestor.
Nonsense and common sense
Biologists reason that all living organisms on Earth must share a single last universal ancestor, because it would be virtually impossible that two or more separate lineages could have independently developed the many complex biochemical mechanisms common to all living organisms.
Convergent evolution describes the independent evolution of similar features in species of different lineages. The wing is a classic example of convergent evolution in action. Flying insects, birds, and bats have all evolved the capacity of flight independently. The ancestors of both bats and birds were terrestrial quadrupeds, and each has independently evolved powered flight via adaptations of their forelimbs. Although both forelimb adaptations are superficially "wing-shaped," they are substantially dissimilar in construction.