Part 13 of 22 in Evolution Basics

Evolution Basics: The Placental Revolution

In the last post in this series, we discussed the origin of the eutherian “body plan” as a gradual shift away from an egg-laying reproductive strategy to a placental one. While crown-group placentals (i.e. modern-day eutherian species, their last common ancestral population, and all species descended from that common ancestral population) are remarkably diverse (think whales and bats, for example), these species are thought to have arisen from a common ancestral population that lived approximately 65 million years ago – either just prior to, or just after, the extinction of (non-avian) dinosaurs.

From early eutherian to the crown-group placental ancestor

The earliest-known eutherian stem-group species, Juramaia sinensis, is dated at ~160 million years ago (in late Jurassic period). The eutherian fossil record from the Jurassic (and the more recent Cretaceous) is sparse: the discovery of Juramaia was significant in that it extended the known range of eutherians 35 million years back from the oldest eutherian then known at the time (Eomaia scansoria, at ~125 million years ago). So, while stem-group eutherians were present from the late Jurassic on, it seems they were not common (and they serve as a reminder that the fossil record is biased towards common, and widespread species). Despite their relative scarcity, this diminutive lineage had found a niche – likely as small insectivores/scavengers – in a landscape dominated by dinosaurs.

Stem group eutherians such as Juramaia and Eomaia indicate that the last common ancestral population of crown-group placental mammals was likely a small insectivore.

All this was about to change, however. About 66 million years ago, an event would alter the course of vertebrate evolution on a global scale: the impact of an asteroid about 10 kilometers (6 miles) in diameter at Chicxulub on what is the modern-day Yucatan peninsula. The impact released the energy equivalent of millions of atomic bombs, caused tsunamis of staggering size, and littered North America with debris. The impact precipitated a mass extinction event that eliminated all non-avian dinosaur lineages: the remarkable tetrapod diversity of flying reptiles (i.e. pterosaurs), fully aquatic reptiles (such as ichthyosaurs) and large terrestrial, non-avian dinosaurs was lost in the (geological) blink of an eye. This event would bring the Cretaceous period to an end, and usher in the Paleogene (these periods are abbreviated as K and Pg, respectively, with the impact defining the K-Pgboundary). In the aftermath, ecological niches lay open – and mammals would step in to fill them.

Crossing the boundary

What is non-controversial among paleontologists is that the post K-Pg fossil record shows a remarkable diversification of placental mammals. The fossil record, however, is not well-suited to resolving events on a fine scale, as we have discussed previously. As such, it is still unresolved if the last common ancestral population of crown-group placental mammals lived before, or shortly after, the K-Pg boundary. What is not controversial, however, is that in either case, crown-group placentals undergo a significant burst of speciation events in the Paleogene:

While the precise timing of the last common ancestral population of crown-group placental mammals is still a matter of scientific debate, the fossil record shows a remarkable diversification of placental mammals after the K-Pg impact at Chicxulub.

The placental radiation: a case study in convergent evolution

The placental diversification in the Paleogene is highly interesting from an evolutionary standpoint since niches that were vacated by reptiles were in many cases filled by placental species – species that were shaped over time to fill those niches. Though the starting point for this diversification was a small insectivore, convergent evolution (a topic we have examined in detail previously in this series) would reshape species over time in ways remarkably similar to how it had shaped reptiles previously. Consider modern-day cetaceans (whales, dolphins, and porpoises) and ichthyosaurs – both groups show exquisite adaptation for a fully aquatic lifestyle, with flippers, a streamlined body shape, and a tail for propulsion, among other features. While cetaceans and ichthyosaurs are both tetrapods, and thus had tetrapod features in common as ancestral characteristics, these lineages were shaped independently from non-aquatic ancestors to a similar overall form (with some differences, reflecting their distinct trajectories – such as the up-and-down motion of a horizontal tail fluke for propulsion in cetaceans compared to the side-to-side motion of a tail in ichthyosaurs, or the fact that ichthyosaurs had four flippers, whereas cetaceans have almost completely lost their hind limbs). And as we expect for any major transition over time, the terrestrial mammal-to-cetacean transition is attested to by a robust fossil record of stem-group “whales,” showing the gradual loss of hind limbs, the movement of the nostrils to the top of the head to form a blowhole, and other features characteristic of modern cetaceans. (For those interested in this fascinating chapter in the evolution of placental mammals, I have written a short series on whale evolution that might be of interest.)

Other examples of convergence could be cited – eventually large, terrestrial placental mammals would arise and take on herbivore and predator niches previously held by dinosaurs, and bats would converge on powered flight using membraneous wings, as pterosaurs had done millions of years before – though flying mammals would share the skies with the one dinosaur lineage to survive the K-Pg impact – avian dinosaurs (i.e. birds). While these convergences may have (eventually) appeared in placental mammals, the K-Pg impact and the subsequent extinction of non-avian dinosaurs certainly played a large role in the timing and extent of mammalian evolution at this time.

Enter the primates

The placental diversification in the Paleogene is also when the earliest-known primates enter the fossil record. Since primates are the group to which humans belong, it is not surprising that much effort has been made to shed light on the origins of this group.

Previously, we examined the rapid diversification of placental mammals following the extinction of non-avian dinosaurs (caused by the asteroid impact marking the Cretaceous – Paleogene boundary 66 million years ago). Not long after this event (geologically speaking), primates enter the fossil record. Given that our own species is nested within the primates (a fact first recognized by Linnaeus in the 1700s), the origin of this group has been of considerable interest to paleontologists.

Presently, the oldest-known primate is a tiny creature from the middle Paleogene of what is present-day China. This tiny primate, Archicebus achilles, is a crown-group haplorhine, that lived about 55 million years ago. Haplorhines are a monophyletic group that includes present-day tarsiers, and anthropoid (or “human-like”) species: New – World monkeys, Old – World monkeys, and great apes – including humans. Tarsiers and anthropoids are more closely related to each other than to lemurs, another primate group with present-day species:

Phylogenetic relationships among crown-group primates, showing the position of theArchicebus achilles as a stem-group tarsier close to the base of the primate tree.

Beyond its age and current status as the oldest-known primate, Achicebus achilles is also noteworthy in that it appears to be very close to the last common ancestral population of the crown-group tarsier and crown-group anthropoid lineages, though it appears more closely related to tarsiers than to the anthropoid lineage (i.e. despite being close to the last common ancestor of tarsiers and anthropoids, Archicebus is best understood as a stem-group tarsier). The features of Archicebus and its location close to the base of the primate lineage suggest that last common ancestral population of primates was similarly small in stature, arboreal (tree-dwelling), and fed on insects. The location of Archicebus also supports the hypothesis that primates first evolved in Asia, and later migrated to Africa (where, as we will see in an upcoming post, the lineage leading to humans would evolve).

Goodbye to GULO

It is at the base of the haplorhine tree that an event occurs that we have previously discussed as strong evidence for human – ape common ancestry, and an event that many Christians who have examined the evidence for evolution are familiar with.  One shared feature of haplorhine primates is their curious inability to make their own vitamin C, forcing them to acquire it from their diet. This is unusual – the majority of mammals (including non-haplorhine primates) are able to make vitamin C starting from the basic dietary sugar glucose. The genetic reason for this deficiency has been determined: in haplorhine primates, the enzyme that performs the last step in vitamin C biosynthesis (called L-gulonolactone oxidase; abbreviated as “GULO”) is missing. Despite this lack of enzyme function, however, the mutated remains of the gene that codes for the GULO enzyme is found in these primates. Since this deficiency affects all haplorhine primates, the simplest explanation is that this enzyme function was lost only once, prior to the divergence of the haplorhine crown group:

In order to appreciate the molecular details of the loss of GULO in the haplorhine lineage, let’s briefly review gene structure and function, something that I have discussed in more detail elsewhere:

Genes have a typical structure (obviously simplified here somewhat). First off, there is the actual DNA sequence that specifies the protein product sequence (the so-called “coding sequence”, shown in blue). This sequence is usually broken up into segments in mammalian genes, and these sequences are spliced together when the DNA sequence of the gene is transcribed into a “working copy” called mRNA – a short duplicate of the code that can be used by the cell’s machinery to actually build the specified protein.

The technical name for a gene segment that is spliced into the mRNA is an “exon” – and functional GULO genes in mammals have twelve exons. When comparing the sequence of the GULO gene in great apes (which are crown-group haplorhines), we see that many of the exons of the GULO gene have been deleted entirely, completely removing the possibility of function. In humans, for example, only exons 4, 7, 9, 10, and 12 remain – and chimpanzees and macaques have the same pattern:

A schematic diagram of a functional GULO gene compared to the state observed in several crown-group haplorhines for which genome sequence data is available. The same seven exons in humans, chimpanzees, and macaques are deleted from the GULO pseudogene, destroying the function of the enzyme.

It is not likely that all of the deletion events that removed these exons were the original mutations that removed the function of this gene in the haplorhine lineage. Rather, these deletions likely took place after the original loss of GULO function, when additional mutations to this (now nonfunctional) sequence would have no further consequence to an already defective gene. The shared deletions we see in great apes, however, did take place before these species went their separate ways – and were inherited by the resulting species in this group, in exactly the way we have discussed previously in this series, when we spent several posts likening genomes to “copied texts” that accumulate errors over time.

Shared mutations – a perennial challenge to antievolutionary views

While observing defective genes with identical mutations in nested groups of species is easy to understand from an evolutionary viewpoint, this issue remains an extremely difficult problem for antievolutionary organizations to explain. Indeed, the only recourse is to try to argue that the identical mutations we observe across numerous species were not inherited from a common ancestor – and thus must have occurred independently in each species. The probability of identical mutations occurring multiple times in independent species is of course vanishingly small, to say nothing of such events producing the precise phylogenetic pattern predicted by all other lines of evidence.

So, despite the unfortunate loss of GULO – and a consequent requirement for dietary vitamin C – the anthropoid lineage would continue to expand and diversify. In the next post in this series, we’ll continue to examine the diversification of this group, and the appearance of our own group within it: Homo.

Dennis Venema
About the Author

Dennis Venema

Dennis Venema is professor of biology at Trinity Western University in Langley, British Columbia. He holds a B.Sc. (with Honors) from the University of British Columbia (1996), and received his Ph.D. from the University of British Columbia in 2003. His research is focused on the genetics of pattern formation and signaling using the common fruit fly Drosophila melanogaster as a model organism. Dennis is a gifted thinker and writer on matters of science and faith, but also an award-winning biology teacher—he won the 2008 College Biology Teaching Award from the National Association of Biology Teachers. He and his family enjoy numerous outdoor activities that the Canadian Pacific coast region has to offer.