This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we delve deeper into phylogenetic thinking and begin to examine the origins of our own group – the vertebrates.
In our last post, we introduced the distinction between “stem group” and “crown group” organisms, and discussed how the “arthropod body plan” – i.e. the body plan of crown group arthropods – arose through a gradual process. In this post, we’ll explore the origins of vertebrates, the group that would ultimately give rise to mammals such as our own species. Like arthropods, vertebrates make their first appearance in the fossil record in the Cambrian; and, as we have seen for arthropods, the vertebrate “body plan” shows signs of having been assembled over a lengthy period for exactly the same reasons – we see “stem group” organisms with only a subset of the traits characteristic of crown-group organisms. Before we discuss the origin and diversification of the vertebrates, however, let’s expand on a point we touched on briefly in the last post – the expectation that stem group species arenot the direct ancestors of crown group species, but nonetheless are informative about the evolutionary lineage leading to the crown group.
Stem groups, direct ancestry, and transitional forms
As a scientist, reading popular news reports about biological discoveries is often a painful experience. Most news stories about fossil discoveries, for instance, are plagued with misconceptions. One very common misconception is that paleontology is the search for thedirect ancestors of present-day organisms. While paleontologists are certainly interested in what the ancestors of present-day organisms looked like, paleontology is not well suited for finding direct ancestors. Counter-intuitively, however, this does not prevent scientists from learning a great deal about lineages leading to a present-day crown group. Let’s use human family trees as an analogy to explain why this is the case.
With “Venema” as a last name, it won’t be a surprise that my ancestry is rooted in the Netherlands. If I were to travel to the Netherlands and visit a medieval graveyard, the chances that any particular grave would hold the remains of one of my direct ancestors would be tiny. On the other hand, studying the remains of anyone in the graveyard would be highly informative about my ancestry, because nearly all individuals found there would be (fairly) close relatives of mine. In other words, I would share an ancestor in common with them – though some number of generations back. If I were to call my own immediate family a “crown group,” then these relatives that branch off my direct lineage a few generations back would be analogous to “stem groups” – and studying the characteristics of these “stem group” relatives would be an excellent way for me to learn about my own lineage, even if I knew nothing about it directly, since I would be studying close relatives of my direct ancestors.
So too with fossil species: the probability that any given fossil species is a direct ancestor of a modern-day species is vanishingly small. Fossilization is a highly infrequent event – fossils spaced 10,000 or even 100,000 years apart would be considered to be nearly simultaneous in their timing from a geological perspective – and the chances of such an infrequent event preserving a direct ancestor is highly unlikely. On the other hand, the probability that the fossil record preserves relatives of modern-day species is quite good, though these relatives might be fairly distant ones (certainly much more distant than the relatives I might find in a Dutch cemetery).
Let’s briefly return to the diversification of arthropods to illustrate what we mean. In the phylogeny below, the direct lineage of crown-group arthropods is outlined in blue, and various stem-groups branch off this lineage along the way. Studying these stem-group species allows us to infer what characteristics were present at different time points along the direct crown-group lineage, and the order in which the characteristics were acquired.
For example, because we see stem-group arthropods with specialized appendages and body segments (but without a hardened exoskeleton) we can infer that these characteristics arose first on the lineage leading to the crown-group. Note well – these stem-group species (X1, X2 and X3 on the diagram above) are not ancestors of crown-group arthropods, but relatives. Put another way, they arenot transitional forms leading to the crown group. They are, however, species that give us information about the actual transitional forms on the direct crown-group lineage. In this way, the stem-group species display transitional characteristics – the stepwise accumulation of traits that we use to define the crown group.
Another point worth mentioning here is that because stem-group species are not ancestors of the crown group, there is no expectation that they will be older than the last common ancestral species that ultimately gives rise to the crown group. For example, the stem-group species in the diagram above (X1, X2 and X3) are all species found in the fossil record alongside crown-group species. At the time point marked by the dashed red line, for example, it would be no surprise to find additional species that were stem-group species with one, two or three crown-group characteristics. Once lineages separate, they are independent of each other.
(As an aside, it is fairly common to see misunderstandings of these concepts in the popular press and in Christian antievolutionary writings. Often, the above phylogeny would be interpreted as species X3 being the direct ancestor of species X2, which is in turn the direct ancestor of species X1, which in turn is the direct ancestor of the modern-day species. At times this (erroneous) expectation is even specifically derided as impossible, since (for example) species X2 appears in the fossil record later than species X1, even though species X2 is “supposed to be the more primitive species.” The misunderstanding arises from (a) the expectation that evolution is a ladder-like progression directly towards present-day species rather than a branching tree of related species and (b) the expectation that the fossil record shows us the preconceived direct progression of transitional forms rather than an infrequent sampling from various parts of the branching tree.)
The origins of vertebrates
With these concepts in place, then, we are, at last, ready to delve into the Cambrian origins of our own group – the vertebrates. As you now understand, seeking to understand the origins of the defining characteristics of vertebrates is to look for stem groups on the vertebrate lineage and examine their characteristics.
Vertebrates are a monophyletic group nested within a larger group known as chordates, which means that understanding vertebrate evolution requires us to examine chordate evolution first. Chordates are defined as organisms that have (1) a hollow dorsal nerve cord, (2) a rod-like, flexible structure called the notochord, (3) a pharynx (with pharyngeal openings, sometimes referred to as “gill slits”) and (4) a tail that extends past the anal opening (a “post-anal tail”). Vertebrates have all of the features of chordates, but add others, such as a (5) brain encased within a skull, and (6) a backbone that replaces the embryonic notochord later in development. As you might expect, there are several Cambrian stem-group species on the vertebrate lineage that allow us to see how the defining vertebrate characteristics were assembled over time – a topic we will explore in our next post.