Concerning common descent, Charles Darwin suggested, and many evolutionists today agree that life started as one simple uncomplicated kind of cell. Over time as the population grew, some populations of cells became more and more different from other populations. Over long ages these developed into different kinds of creature. These various new groups kept developing more different groups. Darwin pictured this as a tree of life with branches representing animal groups which keep developing into ever more numerous and ever more sophisticated body plans. The closer the branches are to each other, the more similar the organisms should be to each other and the shorter the time since they diverged.
The assumption of common descent requires simple animal remains in Precambrian rocks. And as we saw, evolutionists are forced to explain the lack of such ancestors, and they do this in different ways. See Cambrian explosion, molecular clock.
Assumptions about horizontal gene transfer have greatly complicated any conclusions about descent with modification (evolution) and cladistic methods of calculating evolutionary relationships have similarly drastically resulted in very different organisms clumped together as sharing a common ancestor. Thus, new plots of the tree of life often look very different from each other and from former popular representations of the phylogenetic tree. Was there really a line of descent which can be represented by a tree of life and can scientists actually tell us what it was?
A recent essay entitled: The Past, present and future tree of life by Cedric Blais and John M. Archibald in Current Biology 31, R314-R321, April 12, 2021, demonstrates why the tree of life concept (phylogenetic tree) is in serious trouble. They start out by mentioning the debate as to whether the history of life can be best represented by a “tree” or a “network”. On this issue they firstly declare: “The issues are not merely empirical but also pragmatic, hinging on what scientists value, what they research and the tools they need. Data alone cannot resolve such a debate – a fact that provides an opportunity to rethink our objects of study and methods, and to experiment with new possibilities. We must not ask which is the truer picture of evolution – trees or networks – but rather what is it that we are trying to picture in the first place, and why.” [p. R314] The issue is apparently pragmatic, not a search for truth.
The authors survey the factors which have caused many to question the usefulness of the tree of life concept. For a start, they declare: “the very existence of endosymbiosis and endosymbiotic gene transfer challenges the tree of life, as they testify to the possibility of lineage fusions deep in evolutionary time.” [R317] The main problem is, of course, lateral gene transfer (HGT) which is a theoretical device to rescue evolution theory from data which do not fit evolutionary expectations. Thus, they declare: “More than a mere distraction for verticality, lateral gene transfer is a driving force of prokaryotic evolution. Life is indeed a network.” [p. R317]
Recent work raises the question of whether there really was an evolutionary line of vertical descent. The specialists think they may be able to distinguish a “statistical tree of life” which does not represent any specific history. “The continued use of the tree of life for classification is thus as much a reflection of its practical convenience and historical and cultural inertia as it is a commitment to natural classification.” [R318] The authors are skeptical of the reality of the tree of life because “Extracting phylogenetic signal from genomic data can be difficult, and much of the evidence for ancient relationships is inconclusive at best.” [R 319]
The tree of life may not be evident from the data but in a pragmatic sense it “is still a source of insight into evolutionary history.” [R319] The upshot of the issue is that these biologists admit that there is no clear evidence that there ever was a line of descent from simpler ancestors to more complex creatures. But they like the idea for its evolutionary implications! That is not good enough for a supposed scientific explanation for the relationships of all life forms.
Christians see the logical relationships of organismal characteristics as reflecting the sovereign purposes of God. The reason that we can separate organisms into a hierarchical organization is because God demonstrates his wisdom and logical character by designing life this way.