Evolution Basics: The Placental Revolution, Part 1

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November 21, 2013 Tags: Genetics, History of Life

Today's entry was written by Dennis Venema. You can read more about what we believe here.

Evolution Basics: The Placental Revolution, Part 1
Photo credit: Dallas Krentzel (Creative Commons Attribution 2.0 Generic)

Note: 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 discuss the rapid diversification of placental mammals following the Cretaceous – Paleogene (K-Pg) mass extinction approximately 66 million years ago.

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.

 

Phylogeny
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-Pg boundary). 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:

 

Phylogeny
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. In the next post in this series, we’ll begin to trace characteristics of our own species as they are revealed by stem groups on our own lineage.

 


Dennis Venema is Fellow of Biology for The BioLogos Foundation and associate professor of biology at Trinity Western University in Langley, British Columbia. His research is focused on the genetics of pattern formation and signalling.

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Roger A. Sawtelle - #83593

November 21st 2013

Dennis wrote:

species that were shaped over time to fill those niches.

The argument I have with Darwinism is two fold.  First is that it basically ignores how the process of Natural Selection works, which reduces it to a theory of Variation.

This means in Darwinian theory that Variation powers evolution, which isa not the case.  Dennis indicates that in this essay that it is ecology that powers evolution in that it is changes in the environment, most notably the astroid strike that caused the extinction of the dinosaurs, that led to the rule of the mammals.

Since this is common knowledge even among most school children, what is amazing is that this has not been accepted as a basic change in Darwinian Theory, which remains Malthusian, rather than ecological.  It seems to me that the anti-ecological point of view of Dawkins must play a role here since he is very zealous in maintaining the purity of Darwinian theory.  That is why it is important to reject Dawkins and his concept of the Selfish Gene.

The second issue is closely tied to the first.  In the essay their is a reference to another essay on extinction.  In it Jeff Schloss says that evolutionary biologists have accepted cooperation as well as competition as a strategy for survival. 

This points to an eclectic view of evolution.  In other words we add on new ideas if the old ones don’t always work.  This is not the proper way to do science.  If the old ideas such as Malthus, don’t work, then they must be rejected for those which do work. 

The eclectic way allows old views to exist which don’t work, while hindering the exploration of new views, which must exist in uncertain harmony with the old.

The fact is ecology and thus evolution works by seeking mutual benefit for all concerned as far as this is possible.  It is forward looking or teleological, contrary to the Darwinian view. 

Evolutionary biology when it resists the ecological point of view by tossing the environment into an eclectic Darwinian mix makes for bad science.          


sy - #83608

November 22nd 2013

Roger

I think it is pretty safe to say that Darwinism does not ignore the process of natural selection, since that is pretty much what Darwin himself concentrated on. After all, while he knew about variation, he knew almost nothing about its source. What has happened since Darwin, with the explosion of genetics, is that we now know a lot about the sources of variation, and perhaps you are referring to the fact that some people are concentrated on the genetic mechanisms for variation, more than on natural selection. But even neoDarwinism is focussed heavily on selection, (read The Ancestor’s Tale, the classic by Dawkins), so I dont think your complaint really applies to Darwinism.

A great deal more attention is being paid to ecological processes in the formation of variation, as the work of people like James Shapiro and Carl Woese, Barbara Wright and others is becoming better known.


Terrance - #83609

November 22nd 2013

Further to the discussion about whether or not to allow comments, is there no way that Biologos can limit the amount of comments made by one individual? Roger A Sawtelle comments on each post almost as soon as it is up and then just proceeds to spew forth the same opinion over and over again. It is invariably completely irrelevant to the topic of the post (except for a couple of quotes he can pull out which he can argue against in proclaiming his views on ‘ecological evolution’) and does not further the discussion in any way. He simply wants to bang on endlessly about the same thing; promoting his flawed understanding of evolutionary theory. This has been pointed out to him many times, yet each time he simply ignores attempts to correct his misunderstandings and continues repeating his tedious mantra. Some might say that we should just ignore him (which almost everyone now undoubtedly does), but why should the comments section of each post be clogged up with untold posts from just one person in a never-ending crusade to promote the same (erroneous) view?


beaglelady - #83611

November 22nd 2013

True, it’s sad to see every thread getting hijacked.  Maybe a rule of no more than one troll post per day would be a good idea. At the Pandas’Thumb they do a much better job of moderating the discussions. After one or two posts, trolls are sent to the “bathroom wall.”   There they are free to post whatever they want, and a good discussion doesn’t get derailed.


beaglelady - #83604

November 21st 2013

Excellent as usual, Dennis!!


Roger A. Sawtelle - #83616

November 22nd 2013

To my critics:

I apologize if you are tired of my concerns about evolution and ecology.

I am not the first person to say that there is a conflict between these two important scientific disciplines.  Frank Perry, M.D., published the widely read book, Darwin’s Blindspot in 2002.

He had two concerns, 1) Darwin’s atomistic conflict driven view of Natural Selection, or the Selfish Gene is bad ethics, and 2) it is bad science because it ignores the contributions of ecological thinkers such Lynn Margulis who see Symbiosis as the basis of ecological and evolutionary change.      

He is not alone Dennis Noble and others have made similar critiques of neoDarwinism. 

Now I hear some say that neoDarwinism has made adjustments to ecology.  It has accepted some Margulis’ work, but not her theories.

The key for me is Symbiosis.  Is Symbiosis the basis of evolutionary and ecological change?  I say Yes and I hope you agree and we can work together.

If you say No, point out a good example and we can discuss it.

 

 


Bobsie - #83618

November 22nd 2013

I think the point is your objection was addressed in a lesson months ago. You may not have accepted it but it was addressed. And hearing what you said initially was interesting. Now, it’s just an annoying broken record. Showing desperation does not enhance your position. 

Just my unsolicited opinion. 


beaglelady - #83631

November 24th 2013

You would not believe how long this has gone on!


Roger A. Sawtelle - #83670

November 26th 2013

Bobsie,

Thank you for your response.

Please tell which lesson you are refering to. 

People are telling me that I am mistaken, but no one has yet to present a valid reason why. 

I have asked for evidence and BeagleLady refered me to some very interesting studies.  However it is clear that they supported my position.

Again do you hold that conflict driven, atomistic Natural Selection drives evolution as Dawkins maintains or not?     

If so, where is the evidence? 


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