"Tetrapods evolved from fish" might be intended to imply that the last common ancestor of a fish clade and a tetrapod clade was itself a fish. However, this is not strictly true, because while the last common ancestor of both clades may have had more obvious physical similarities to living fish than to living tetrapods, it was not identical to any living organism (fishlike or otherwise). Both lineages—the one leading to living fish (e.g., goldfish) and the one leading to living tetrapods—have been evolving independently for hundreds of millions of years, and during that time, evolution has not stood still on either the fish or tetrapod branch. Over this period, all aspects of fish physiology and the fish genome have changed, though perhaps in ways that are not obvious to the human eye. Thus, it is not accurate to say that the common ancestor of both fish and tetrapods was a fish. The best you could do would be to say that the common ancestor had a body form and ecology that were more similar to that of living fish than to that of living tetrapods.

Another problem with ladder thinking is that even with such clarifications, it is still easy to make errors of reasoning. For example, suppose you are told that goldfish have body outgrowths (in this case, fins) with cartilaginous structures called rays. You are also told that tetrapods lack rays in their body outgrowths (limbs). If you took a progressive view, you might assume that tetrapods lost their rays during the course of evolution. In reality, however, the common ancestor of both tetrapods and goldfish lacked rays; thus, rays evolved along the lineage that leads to goldfish. In this case, if you had assumed that the ancestor species had rays, ladder thinking would have led you astray. In order to avoid such mistakes, it is best not to make statements such as "tetrapods descended from fish," or at least to do so with the clear understanding that "fish" is referring only to body form and ecology and not to any other features of living fish species.

Tree thinking teaches us that all living organisms are equally distant in time from the root of the tree of life and therefore all are equally advanced. Thus, in the eyes of evolution, a human and a bacterium are equally derived. Although one of these organisms is certainly more morphologically complex than the other, both organisms are remarkable in that they are the product of parents that successfully and repeatedly gave rise to offspring over an unimaginably long time span (at least 3 billion years).

This egalitarian view of life may seem hard to swallow. However, before you reject this idea, consider how the world might look if you were a ladder-thinking bacterium. If that were the case, you would certainly be struck by all the amazing molecular adaptations that your ancestors had accumulated to make you and your kin so successful. You would probably point to a human and note that within its body, there are more bacterial cells than human cells, thereby proving the superiority of bacteria over lumbering eukaryotes. You would likely consider bacteria to be the pinnacle of creation and the rest of the planet's organisms to be evolutionary rejects. On the other hand, if you were a tree-thinking bacterium, your view of life's tapestry would be just like that of the tree-thinking human—in this instance, you would appreciate that all living things are equally amazing products of over 3 billion years of evolution. Thus, tree-thinking not only provides important practical tools for organizing knowledge of biodiversity and for reconstructing evolutionary history, but it also provides a clear and unbiased metaphor for evolution at large.