According to evolutionists...

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 scope. Furthermore, several researchers have proposed that early life was characterized by rampant horizontal gene transfer, leading some to question the monophyly of life.

However... It wasn't Charles Darwin who invented the idea that all living things supposedly descended from one universal ancestor. His grandfather Erasmus Darwin mentioned it before him. Hindus already had present notion of common ancestry between humans and animals. Old beliefs die hard. According to mainstream science organizations like NCSE: common ancestry forms the core of evolutionary biology.*

Common ancestry is the belief that all life forms originate from one first life form. It was popularized by Darwin in the 19th century. But as the evolutionary scientist says in the article above, until this day there haven't been any true scientific evidences for this mythical idea. Any true scientist will immediately admit that such an idea cannot even be scientifically proven. Therefore evolutionists invented mathematical models to seemingly support it. This resulted in today's neo-Darwinism, the "latest and greatest" of evolution theory.

Charles Darwin:
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.

Darwin popularized the belief in common ancestry. He based his belief on homology. But using similarities in order to imply a relationship is part of metaphysics and has nothing to do with science. Science can never show a relationship between two distinct life forms. All we truly know is that different life forms are unique and do not morph into each other or into other life forms. Nevertheless this tree thinking has become a mental construct within the world of evolution...

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.*

The different life forms in the supposed evolutionary tree of life are a fact. The lines or branches connecting them are based on the preconceived belief in common ancestry and macroevolution. The timeline in which it supposedly happened is based on the unprovable philosophy uniformitarianism. What is missing between the artificially connected life forms are the so-called transitional forms. These are imagined by evolutionists. Evolutionists make use of metaphysics and circular reasoning to support their view. Today this metaphysical tree of life has become part of evolutionary paradigm paralysis. And now after many years of mainstream science indoctrination many people simply take that baseless faith for granted and even go as far as to call it a fact.

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.*

Here we see the typical kind of evolutionary reasoning. Because life forms show similarities, in for example their genetic makeup, evolutionists reason that all life forms are related through evolution. This is metaphysics and a twisted way of reasoning. It is like saying that all computers run on software and therefore they are all supposedly related by means of evolution. This is not only absurd, it is completely false. Of course everybody knows that computer science is a result of intelligent design.

The coming into existence of life is a complete mystery to science but it happened at least once. Evolutionists refuse to believe that more than one life form came into existence "like the first one". That would be "too impossible", but "the first one did". The Cambrian explosion shows the sudden appearance of practically all phyla of complex life at a relatively short timespan in history. And contrary to the statement in the Wikipedia definition of the history of life convergent evolution does force evolutionists to believe in the independent development of complex biochemical mechanisms...

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.*

So, in the article about the history of life the authors said that the independent development of complex biochemical mechanisms is "virtually impossible". Then with convergent evolution they state that the independent evolution of similar complex biochemical mechanisms in different species "supposedly happened". Another contradiction showing the weakness of evolution theory. It simply makes evolution theory unfalsifiable because, whatever is the case, according to evolutionists "evolution did it". It's not the God-of-the-gaps, but the Evolution-of-the-gaps. Evolutionists made up many theories contradicting each other. The thing that surely evolved over time is evolution theory itself.

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Craig Venter:
The tree of life is an artifact of some early scientific studies that aren't really holding up, ... so, there is not a tree of life.**