In this talk, the lack of structure that I mentioned earlier is very apparent, as we go through three not-directly-related topics.
First, we will look at two select examples of vertebrates that underwent spectacular changes in the Tertiary. Second, we will look at the Ice Ages. Third, we will look at what will happen if current rates of anthropogenic global warming do not subside.
The reason for this disjointed set-up is because many events happened in the Tertiary, both from a biological and a geological perspective, and it is useless to run through them in 45 minutes while blurring out all the details. Instead, I chose to do a “case study” approach.
The first of the groups we will look at are the birds. Specifically, I will show you how we know that birds are dinosaurs – this is now common knowledge, but some short googling has shown that “skeptics” still abound, so I might as well set the record straight. The way to go about this is to simply draw a cladogram with the birds at the top, and see which characters from their ancestors the birds have kept, using the picture of Archaeopteryx as a visual guide. While this is a good pedagogical technique, it does stink of teleology, so I will say this right now: there is no plan along which the birds evolved; we’re simply using our hindsight here.
Anyway, the pattern that will emerge out of this exercise is that of mosaic evolution, that birds are basically cobbled together from the parts that evolved earlier in their dinosaur ancestry.
The basalmost character, shared with the pterosaurs as well, is the mesotarsal ankle, i.e. that their tarsi (the bones before the toes) form the ankle, hence why we say that birds walk on the tips of their toes.
Diagrams source: Makovicky, P. J. & Zanno, L. E. 2011. Theropod Diversity and the Refinement of Avian Characteristics. In: Dyke, G. & Kaiser, G. (eds.). Living Dinosaurs: The Evolutionarx History of Modern Birds. Wiley Blackwell.
Going further up the tree (naming all of these nodes is an exercise in futility), we start getting more and more dinosaurian characters being evident in the birds. The loss of pentadactyly (i.e. having more or less than five fingers), the advent of extensive pneumatisation (hollowing out of bones to make them lighter), especially in the sacral region, the loose 1st toe (in the picture, the stub that sticks out) and, most importantly, a new bone called the furcula.
The furcula is in the chest area and serves as an extra site for forelimb muscle attachment. In the birds, it is what gives them the extra power needed to achieve flapping flight.
Going further up the tree, we get the appearance of the maxillary fenestra, a characteristic hole in the skull in the maxillary (upper jaw). Other changes include a strap-like scapula (shoulder bone) and a broad dorsal atragalus (ankle; remember that in the dinosaurs and birds, the ankle is high up).
- The pneumatic ectopterygoid, one of the skull bones becomes hollow.
- 3 tympanic systems for better hearing and balance (modifications in the ear)
- A promaxillary fenestra, another hole in the upper jaw, hinting at more efficient construction.
The fused semilunate carpal refers to the carpal (wrist) consisting of fused bones forming a semilunate shape.
The reduced tail is most recognisable: compare the tail of a sauropod or a T-Rex with that of a chicken or pigeon. This trend started with the maniraptorans.
The retroverted pubis is characteristic of the birds; see “Ornithischia” and their convergent evolution of the same pubis structure.
The feathers from this point on become asymmetric, hinting that they get more porminent roles; all flight feathers are asymmetric, but flying was still not achieved at this point.
Finally, their ulna (lower arm) is subdivided into several regions.
Besides all the previous characters that were accumulated in their ancestry, the birds (Aves) have one characteristic feature: their forelimbs are longer than their hindlimbs. This may not be readily apparent, since the forelimbs are always folded. But the functional morphological purpose is clear: the longer forelimbs are there for flight.
The other vertebrate groups went about achieving flight in other ways: pterosaurs extended their fourth finger, while bats elongated all their fingers and spread a membrane over them. It just goes to show that functional convergence is vastly different from morphological convergence.
The other vertebrate group I want to focus on is the whales and dolphins (Cetacea), because not only are they cool examples of macroevolution seen in the fossil record, but also because in recent years, our knowledge of whale evolution has been revolutionised by new findings that have both pushed back the timing of the origin of whales (see diagram) and also changed our view of how whale evolution took place.
Diagram source: Uhen, M. D. 2010. The Origin(s) of Whales. Annual Review of Earth and Planetary Sciences 38, 189-219.
As you can see on the left, the cetaceans are most closely related to the hippoes, forming the Whippomorpha clade. More distantly, their living relatives include the ruminants (cows) and Suiformes (pigs).
As for the cetaceans themselves, their autapomorphies (diagnostic characters) are all in the skull and teeth, as shown on the right.
The Cetacea refers to all animals that share the above autapomorphies and includes the fossils. Modern whales belong to the Pelagiceti (the pelagic whales) and, more narrowly, the Neoceti (Recent whales). For a timeline, by the end of the Miocene, modern whales had evolved and were coexisting with the pelagicetes, which died out at the end of the Miocene,
Modern whales are split into the Mysticeti (balleen whales) and Odontoceti (toothed whales).
Of course, it’s been known for a long time that whales and dolphins are related, and that they are mammals. The striking thing about them is that they are the only mammals to have reverted to a fully aquatic lifestyle (seals and their ilk are not quite there yet). It was only in the past couple of decades that the fossil record has allowed us to reconstruct how this very important and drastic transition took place, thanks to a range of findings from Pakistan.
We will go through the most significant of these fossils now, in phylogenetic order.
The basalmost fossil is Pakicetus and its smaller cousin Ichthyolestes, in the Pakicetidae family. Besides sharing those detailed skull autapomorphies of the whales, we can easily tell that they are related to them because of the elongated snout, and the teeth.
Importantly, the pakicetids were mostly terrestrial, at most walking in river beds like hippopotami.
Next comes Ambulocetus of the ambulocetid family. Just a cursory look at the skeleton shows that it has undergone some drastic changes in its morphology, both appearance-wise and function-wise, now bearing a ressemblance to crocodiles. This means that it was more adapted to the water than the pakicetids and probably split its time equally on land and in the water – it could do both equally well.
The next step is known only from the skull of various creatures lumped together under the Remingtonocetidae. It is fairly obvious that these are whale-like from the elongated jaw as well as the teeth, which are more adept at catching fish, therefore hinting at a more aquatic lifestyle.
This trend is continued with the Protocetidae. As the name suggests, these are the proto-whales, referring not to phylogenetics, but to functional morphology. These ones were mostly aquatic, as can be seen from the streamlined body shape and the limb extremities, with elongated and spread-out fingers serving as paddles. We imagine them leading their lives like seals: hunting and spending most of their time in the water, coming on land to reproduce or hang around.
The final move away from the land was complete with the basilosaurids, most well-known from Basilosaurus. They form the base of the Pelagiceti taxon. It is obvious that this animal could not have lived on land: the streamlined body shape, the reduced hindlimbs and flipper-like front limbs are typical of the wahles.
In fact, it is now hypothesised that the modern wahles are direct descendants of one basilosaurid group.
Once in the water, whales were free to grow to enormous sizes. Their only threat was the presence of sharks, who were also quite diverse in the Tertiary, and it is possible that the large size of the whales is the result of an ancient coevolution between the two groups, both of which were severely affected by climate changes occurring in the Miocene which led to a decrease in their diversity.
From that extinction, only two whale groups survived: the balleen whales (above) and toothed whales. Balleen plates are present in these whales instead of teeth. These are made of keratin and grow from the upper jaw down in parallel rows. They serve to filter small animals from the swallowed seater.
As a sidenote, all cetaceans are highly-intelligent animals, even relative to other mammals. They can communicate across ocean distances using sound waves, and some researchers have even suggested that they have a language. Dolphins are well-known for their mimicry abilities and it is very likely that they have language. I’ve also already written a bit about their sleeping.
The map above shows Central Europe and the lines represent the maximal extent of several ice sheets of different ages.
The Pleistocene is the period of time from ~2.5 Ma, and since then, the world has been in the grips of several recurring Ice Ages.
The root cause of these Ice Ages are the Milankovitch Cycles, of which there are three of variable frequency.
The first relates to the eccentricity of the Earth’s orbit around the Sun. The Earth doesn’t orbit it in a perfect circle, it is more like an oval, and the centerpoint of this oval is constantly shifting, causing severe summers and extreme winters (see upper diagram). Every 100 ka, a new cycle starts.
The second related to the obliquity of the Earth, i.e. how tilted it is. This cycle repeats every 41 ka, and at its most extreme, causes the Upper Hemisphere to get much more sunlight, while the Lower Hemisphere gets less.
The third relates to the precession of the Earth, i.e. how it’s wobbly on its axis. This cycle is shorter, repeating every 23 ka.
Basically, these all lead to changes in how much sunlight, i.e. energy, enters the Earth’s system. Insolation, i.e. temperature, is what ultimately drives the Earth’s climate (differences in temp. → air currents → oceanic currents → Earth’s climate).
Climate is obviously a huge, very complex system with many interconnections and feedbacks. However, the effect of the Milankovitch cycles is very clearly seen in the geochemical record of the Pleistocene, especially in the more recent stuff. Take the case of the eccentricity and oxygen isotope ratio (a proxy for temperature): the 100 ka cyclical trend is undeniable.
Obviously, changes in climate lead to biotic changes as well. We’ve seen this throughout these lectures. The Ice Ages are unique in that we are still in the middle of them and have the opportunity of studying these changes first-hand.
The Ice Ages are known for the development of megafaunas in the colder higher latitudes, animals that could survive in the frosty tundras. Arguably the most famous are the mammoths.
Their thick fur coat was obviously for insulation. They are relatives of the elephants, but not elephants themselves. You can tell them apart most easily from the tusks (mammoth tusks are curved sideways, elephant tusks are straight), and from the tooth structure.
As a sidenote, one occasionally hears news reports about mammoth DNA and even mammoth cloning. Bits and pieces of mammoth DNA have been isolated, but cloning is nigh impossible.
Gigantism seems to be a theme of the Ice Age faunas. Bears are the products of the Ice Ages, as are the now-extinct giant ground sloth (Megatherium) and giant armadillo (Glyptodon), whose fossils were extensively collected by Charles Darwin in Patagonia.
Another famous Ice Age megafaunal creature is the sabertoothed cat. The development of saber teeth is not unique, many vertebrate groups throughout Earth’s history have had saber-toothed derivatives. However, the functional morphology of the saber teeth is still something of a mystery. Such oversized teeth do not seem very practical even for impaling prey.
Image credit: That shitty movie by Roland Emmerich.
And of course, human success is also largely a product of the Ice Ages. The hominids expanded twice out of Africa, the second time being the radiation of the modern humans (Homo sapiens) due to the climate changes brought about by the Ice Ages. They could spread throughout Asia and Europe, moving from the latter to the American continent across the Bering land bridge. They are the only primates to have conducted such a dispersal event (New and Old World monkeys were separated by plate tectonics, not willful migration).
The reason underlining human success is not only the several morphological adaptations we have – bipedalism, opposable thumbs – but also our sociality. As all great apes, humans have a great propensity for mimcry, as demonstrated by George Carlin, upper left. Humans have also greatly developed the grunts and screeches of their simian ancestors into fully-fledged languages (accompanied by changes in brain and and throat structures, of course). This allows us to communicate together in much deeper ways and craft more meanings. Although sometimes, this doesn’t really work, as demonstrated by Deepak Chopra, bottom left.
Accompanying/causing/a consequence of these is a great increase in intelligence, as evidence by the patriot, upper right.
All of this allowed us to come together, cooperate and live peacefully with each other, as embodied by the principles of the milk-lover, bottom right.
Humans may cooperate with themselves, and they may have chosen several plants and animals to domesticate (thereby ensuring their evolutionary success as long as humans are around), but on the flipside, we have had some detrimental effects on the environment. The diagram above summarises what the main cause of extinction for several Ice Age megafaunas. In red are those killed off by humans, in blue by climate. In brown are the ones where we don’t know.
As you can see, the extinction of the rhinoceros-like marsupial Diprotodon in Australia was caused solely by human overhunting; mammoths were similarly severely affected by humans and done in by climate. It can be reasonably suggested the the South American Glyptodonts suffered a similar fate. Only the Eurasian rhinoceros seems to have gone extinct due to climate, but even there humans played a role.
Diagram source: Koch, P. L. & Barnosky, A. D. 2006. Late Quaternary Extinctions: State of the Debate. Annual Review of Ecology, Evolution, and Systematics 37, 215-250.
Even more grave than our purposeful wiping out of biodiversity are the changes in atmospheric systems and climate we have been bringing about since the Industrial Revolution. Each of the graphs above is pretty self-explanatory, showing very noticeable increases as of the 1900s. Reading the y-axes of these shows that these increases are, without failure, negative: loss of biodiversity, overexploitation, increase in greenhouse gases, increased temperatures, etc.
These changes are much more catastrophic because they affect the Earth as a whole, especially the ones that directly affect climate. The Earth’s biota was in a state of disequilibrium anyway, with the constantly recurring Ice Ages messing around with biogeographical distributions; what we are doing is making these matters much, much worse by throwing in even more chaos.
Diagram source: Steffen, W., Grinevald, J., Crutzen, P. & McNeill, J. 2011. The Anthropocene: conceptual and historical perspectives. Philosophical Transactions of the Royal Society A 369, 842-867.
One thing that is misunderstood about global warming – and that global-warming denialists point to – is the fact that right now, the amount of CO2 in the Earth’s atmosphere is at a minimum. So pumping in some more will not have any effect, since the Earth is “used to” having more CO2.
This is true, but it completely misses the point. The Earth is not an organism and is not adapted to anything. The biosphere is what is adapted, and the modern biosphere, as we saw, arose in a world of low CO2 levels. You may be saying that humans will never pump out enough CO2 to make drastic effect, but logic and data contradict that claim. Logically, fossil fuels are carbon dioxide sinks – they are nothing more than the archives of past atmospheres. By burning them, we are simply releasing and reconstructing that past atmosphere.
And even if one does not care about the overall state of the biosphere, there is a completely selfish reason to care about global warming: we humans, and our societies and civilisations, all evolved adapted to our current climate. All our farming systems and infrastructure depend on the climate remaining somewhat stable. By screwing around with it, we are endangering ourselves.
Diagram source: Berner, R. A. & Kothavala, Z. 1991. GEOCARB III: A revised model of atmospheric CO2 over Phanerozoic time. American Journal of Science 301, 182-204.
Just as the Ice Ages affected species distributions, so will anthropogenic climate change. In the above diagram, PD stands for phylogenetic diversity; it can be treated as a unit of biodiversity. Looking at the right column, we see that in Central Europe, plants, birds and mammals will experience biodiversity drops, except at mountains. Scandinavia will experience biodiversity increases as the fauna and flora migrate there where the conditions are more to their liking.
The opposite can be seen for the lower latitudes, which will be largely arid, Sahara-like landscapes.
You can imagine how these climate changes will affect humans, and especially our food supply: plants do not grow in artificial bubbles.
Diagram source: Thuiller, W., Lavergne, S., Roquet, C., Boulangeat, I., Lafourcade, B., Araujo, M. B. 2011. Consequences of climate change on the tree of life in Europe. Nature 470, 531-534.
And as a throwback to the past, recall the P-T extinction, which was started off by sudden enormous increases in CO2 levels. Marine life was devastated by the resultant ocean acidification: carbon dioxide in water dissociates into carbonic acid, which reacts with calcium carbonate – the material out of which most marine organisms build their shells – and breaks it down, ensuring the death of biomineralisers, which include everything from corals (whose reefs are havens for biodiversity), most molluscs and crustaceans (seafood).
Additionally, the increase in pH affects physiological systems, as does the temperature increase caused by global warming. Also, the melting of glaciers will cause salinity levels to fluctuate. All of these are risks of extinction.
Diagram source: Hoegh-Guldberg, O., Mumby, P. J., Hooten, A. J., Steneck, R. S., Greenfield, P., Gomez, E., Harvell, C. D., Sale, P. F., Edwards, A. J., Caldeira, K., Knowlton, N., Eakin, C. M., Iglesias-Pietro, R., Muthiga, N., Bradbury, R. H., Dubi, A. & Hatziolos, M. E. Coral reefs under rapid climate change and ocean acidification. Science 318, 1737-1742.
The moral of the story is that while we’ve seen that life keeps on recovering after extinction events and that new groups will always fill the ecological roles of the extinct ones, these processes are, for all intents and purposes, chaotic, from random volcanic pulses to a freak asteroid to regular climate fluctuations. There is no doubt that we humans, who have benefited greatly from the latter, will also be killed off by them. But there is no need to hasten that process for the sake of proving to the rest of the biosphere that we are indeed as stupid as we make ourselves out to be.
For excellent debunkings of climate change denialism:
And that’s it for the series. Please excuse that this last part is very disjointed; this entire series of lectures was experimental, with a mixed audience of laymen, students, non-palaeontologist biologists, and professors, and hitting the right note was extremely challenging. I was trying out different styles and techniques so I can be better prepared for future similar lectures.
Research Blogging necessities :)
Uhen, M. (2010). The Origin(s) of Whales Annual Review of Earth and Planetary Sciences, 38 (1), 189-219 DOI: 10.1146/annurev-earth-040809-152453
Thuiller, W., Lavergne, S., Roquet, C., Boulangeat, I., Lafourcade, B., & Araujo, M. (2011). Consequences of climate change on the tree of life in Europe Nature, 470 (7335), 531-534 DOI: 10.1038/nature09705