EEB 122: Principles of Evolution, Ecology and Behavior ?

Lecture?19. The Fossil Record and Life's History

https://oyc.yale.edu/ecology-and-evolutionary-biology/eeb-122/lecture-19

Today we're going to take our third look at the history of life on this planet, and it's going to be about the fossil record and the major groups of life. The first look was major transitions, and the issues involved in them. The second look was how life shaped the planet and how the planet shaped life; so it was a description of the geological theater in which evolution has occurred. And today we'll actually look at the fossil record.

• The?geological eras are actually?defined by fossils, and the coordination of them across the planet is done by matching types of fossils. In the late twentieth century we had radiometric dating that helped a great deal with this.?But the original layout was done with fossils.

• And if we then take everything that's happened since the end-Cretaceous mass extinction, that's called the Cenozoic. So this, the Mesozoic is more or less the Age of Reptiles; the Cenozoic is the Age of Mammals; and we blow the Cenozoic up, this is what we get. We get Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene.

• And the last 10,000 years is the Holocene; that's since the glaciers melted, that's the period we call the Holocene. And roughly speaking the world restocks itself with biodiversity in the Paleocene and Eocene. We have roughly modern levels of biodiversity since the Oligocene, in terms of mammal families and things like that. And most of the mammal orders have their roots in the Eocene and Paleocene.

Now if we look at large-scale events, one of the most interesting is well when did multi-cellular life really get going? And for that the tiny fossils that are preserved in phosphate beds in China are absolutely astonishing.?

• The Cambrian starts at about 550. So this is 20 million years before the Cambrian; we're in the Vendian, we're in the late-Proterozic Era. And a lake, or an inlet, dried up and the salts in it crystallized and they perfectly preserved the algae that were in it. So these are micrographs of microfossils showing multi-cellular algae, and in some of them you can even see the spindles in the mitotic divisions.

• In formations in China of the same age, there are multi-cellular, bilateral animals. These look like early-stage cell divisions of Crustacea. So this is again 20 million years before the Cambrian, and the implication that there might be a Crustacean 20 million years before the Cambrian is a very interesting one, as you'll see in a minute.?

So our molecular phylogenies suggest, looking not at the fossils but at the molecules, that the eukaryotic radiation--so that's before multi-cellularity; this is just the eukaryotic cells making Protista--that was underway about a billion years ago.?

• These microfossils support the idea that many groups may have diverged before the Cambrian, but we have no trace of them in the fossils. We just have this marker; we have these Crustacean-like embryos.

• If that's really true, then the first fossils of large animals, animals that you could see with the naked eye, that had hard body parts, that had endoskeletons or exoskeletons, these things crop up in the Cambrian, and they may simply then be recording the fact that formerly soft-bodied things started to acquire skeletons. So the groups existed before then, they just couldn't be fossilized, and that that may very well have been because of co-evolution with predators.

• The Chinese microfossils were?570 million years ago. And if we just walk out around the Tree of Life, anything that has that branch length from what we think is the origin, was probably there at the same time, even though we don't have a fossil of it.?That's the implication of the molecular phylogeny; it's that everything else that's about that far out from the common ancestor was probably there at the time.?

Most of these other things are single-celled organisms, and we wouldn't expect them to leave fossils. We've got stuff like slime molds and amoebas and euglenas,?algae out here. Those things were probably all there. We just don't have fossils of them. And that's why it's really important to be able to deal with both the molecular phylogenies and the fossils, because they complement each other and they allow a kind of inference that's not available from either alone.?

Now, what happens in the Cambrian? That's when the fossil record really gets going. The idea that there was an explosion of biodiversity in the Cambrian seems to be well supported by the fossils. This is the number of orders that can be observed?of animal groups. By the end of the Cambrian, we start picking up fairly large clades of marine invertebrates that start?getting added on at a pretty high rate.?

And the interesting thing is no major body plans appear in the fossil record, in animals, after that. They do in the plants, but in the animals it's as though there's one burst of diversity, 550 million years ago, and then all the major body plans get frozen, and we don't get new kinds of animals after that. That's kind of a puzzling and not completely solved problem. Why was it that way??

• Now let's take a look at one of these communities. They contain some organisms that are profoundly weird and not?very big.?That's Anomalocaris, and that is an?arthropod-like predator, and it's got some funny sort of quasi-tentacle antennae?and a mouth right here, and it swims around, and it's the biggest, nastiest thing in the ocean.

• In fossil history, things start small and get big. Things start small and have short generation times and short lives, and get to be big and have long generation times and long lives.?And I don't mean by that that the small things are replaced by the big things; the big things add onto them. It's like a community would be dominated initially by small things, and they would continue to be there, but big things would evolve.?

Here are some animal groups first appearing as fossils in the Cambrian.?

• The things in the Cambrian are at least three of the mollusk classes. There?are the chitons, these are the snails, and these are the squids, octopuses and ammonites.

• We get the polychaetes, which are the biggest group of the annelids--the ones that are most familiar?are?earthworms; those are oligochaetes.?There are 43 families of polychaetes. They're a?dominant group in the ocean?for 550 million years.

• We start getting arthropods?and?trilobites. The chelicerates are the horseshoe crabs and the spiders and their relatives, and we start picking up some Crustacea.

• We get the brachiopods, the lampshells, which are still around. There are deep-water brachiopods around the world, but they've mostly been in retreat for a long time.

• And we get echinoderms.?They're the sister group of the chordates, and that implies that the chordates had diverged from the echinoderms?at that point, and they just weren't fossilizing. And we know, from the first fossils that we can get?things like Amphioxus, that if you have a tiny, little, one-inch long, translucent, tadpole-like, fish-like chordate, that's the ancestor of the vertebrates, it's probably not going to fossilize.

• The echinoderms went through an explosive radiation. They made many classes. The different classes of the echinoderms now are things like the asteroids, which are the starfish; the holothuroids, which are the sea cucumbers. There are, I think, six or seven classes currently of echinoderms; but back in the Cambrian there were about twenty-five or thirty. Most of them have now gone extinct. And some of those things that you saw in that earlier picture were extinct classes of echinoderms.?

So for the animals there's this explosion 550 to 500 million years ago in the Cambrian. It's very different for the plants. The plants had a much steadier, more measured evolution of diversity. The major groups of plants arrive later because plants got onto land later. Most of the animal groups, all the animal groups originated in the ocean, but much of plant diversity originated on land; so they had to get onto land.?

• The mosses and the ferns appear in the fossil record in the Devonian, about 400 million years ago.

• The gymnosperms, which is pines and firs and their relatives, they actually are 350-million-years-old. So they appear in the early Carboniferous, and they undergo continuing evolution up to the present day. So they keep getting?diversifying and becoming more sophisticated. But there are recognizable gymnosperms 350 million years ago.

• And when the flowering plants evolve depends on whether you're looking at molecules or fossils. The molecules suggest that it might be as old as Carboniferous-Permian-Triassic; that is, 200 to 300 million years ago. The really solid evidence, of course, is the fossil, at a certain age, and that's in the late-Cretaceous. So you can see angiosperms that are 75-million-years-old in the fossil record.

• The first plant on land might actually have been a liverwort.?It looks a fair amount like algae?that we see in the intertidal zone. It doesn't really look that different in its structure from a marine alga, but it is adapted for living on land.?

And to get onto land this is what you need.?

• If you're an animal, you're going to have to come up with an impermeable skin. If you want to locomote on land, you'll need limbs, and for that you'll need shoulder and hip supports. And if you want to reproduce on land?rather than in the water--which is, of course, what many of the amphibians have continued to do--then you'll need an egg that won't dry out. So you need a shell and an amnion, and this basically happened between the amphibians, and then?everything that came later in the tetrapods.

• If you're a plant, you need an impermeable leaf. That means you need to invent the biochemistry and the developmental biology to make a waxy cuticle. You need a means of gas exchange. So you're going to have to invent all of the neat stuff about stomata and stomatal regulation of carbon dioxide coming in and oxygen going out. And you'll need to have roots, resistant spores; eventually you'll need seeds.

• Perhaps the first stage of coming onto land was the kids went exploring, and then they went back in the water and grew up to be adults. The parents couldn't go into the new habitat because they didn't have limbs that were strong enough to support them.

If we look at the plant radiation, there's a whole series of acquisitions of major elements of what it means to be a plant, and they occur at a pretty steady pace between about 450 and 75 million years ago.?

• Chlorophyll B is on the order of maybe 1 to 1.5 billion years old. You get plant cell structure probably at the level of about a billion years. You get alternation of generations, haploid/diploid generations, coming in pretty early.

• As?you move out of the mosses, and move towards the club mosses, you can see that the water delivery system?of plants?starts to develop. So they're developing roots and they're developing all of the plumbing that will allow water to move and bring nutrients from the roots up into a growing structure.

• Wood starts to develop right about in here, and by the time you get up into the Equisitifolia and the precursors to the gymnosperms, you're getting pretty well developed xylem?and?phloem.?Then the seeds evolved with the gymnosperms; gymnosperm means naked seed.

• This is the radiation of the gymnosperms. The Pinales would be the pine trees, and firs.?And the Gingkoes?are the familiar Gingko trees.?And that makes a clade. And that's where seeds were invented.

• Then as we go up further, we get into pollen grains that have a distal aperture, and then finally we get to the flowering plants.?

Let me just go back and reinforce these two points. Some of the stuff that you need to get on land was developed earlier in the water?for other reasons, and then was co-opted to get you on land, and that's what probably happened with the vertebrate limb. The plants developed much of their diversity after they had gotten onto land. And you can see that they are adding things like vascular canals and water delivery systems and things like that--wood--at a fairly steady pace?between about 450 and 75 million years ago.?

There is a pattern of?continuing extinction, and then re-radiation, and extinction, and re-radiation. The world hold only so many kinds of ammonites. It kind of fills up, and then when it's wiped clean, it?creates?a space for the others to re-radiate. The pattern is consistent with that interpretation.?

• At the end of the Devonian there's a mass extinction, lots of them get cut off, two lineages come through. This lineage radiates, makes a whole lot of different species and families of ammonites. At the end of the Permian, they all go extinct.?This line here manages to get two of them through--two lineages, maybe three--through the Permian mass extinction. One of them goes out in the Triassic; the other radiates. At the end of the Triassic there's a mass extinction. Almost all the ammonites disappear again. One or two lineages get through, into the Jurassic, and at the end of the Cretaceous these two surviving branches both go extinct.

• Mammals started to radiate back in the Triassic. If we were back in the Triassic we might not have called them mammals yet, but they have already split off from other ancestors, and it looks like things like the Monotremes have their roots at about that level in time. So we're looking back about 200 million years.

• That is consistent with the idea that the extinction of the dinosaurs was a necessary pre-condition for the radiation of the mammals. And it looks like a reiteration of the pattern we saw with the ammonites: clean the planet out and make space, and then they can evolve again. So this is really kind of a tetrapod recapitulation of what we saw with the ammonites.?

So what are the groups that are still radiating??

• The beetles are still going like gangbusters. The number of beetles that have been named is I think about 350,000. The number of beetle species that might exist could be on the order of 5,000,000.

• The Diptera, the flies and the mosquitoes, are a young group, and they are still producing new species. Among the mammals, it's the bats that are probably the most impressive producers of biodiversity, along with the rodents. And the place where the bats and the rodents are doing the most of this is in South America.

• In the flowering plants, there is really impressive biodiversity in the composites, the orchids and the grasses. There are about 12,000 species of orchids,?15,000 species of grasses and composites, 30,000 species of?Asteraceae.

What about the stuff that's been wiped out??

• All of those exotic things in the Burgess Shale, they're gone forever, and they've been gone for hundreds of millions of years. The trilobites, the ammonites, the dinosaurs, those are all gone. There was a wonderful group called the glossopterids. They were Jurassic tongue-ferns; they were ferns that looked tongue-like.

• About 10 million years ago, a bunch of tough?North American hoodlums migrated south?over the Isthmus?and ate up everything in South America.?Things like pumas and wolves ate up the South American notoungulates. There were a few things that came north; possums, armadillos came north, but most of it was a movement south. There is?a complex Miocene and Pliocene fossil fauna in South America that's vanished forever.

• In the last 10,000 years, mostly on islands in the Pacific, 25 to 35% of the world's birds have gone extinct. And outside of Africa, most of the Pleistocene megafauna is gone.?

HOX genes?turn an onychophoran into a fly.?Well this is an intermediate step. Basically you take a worm and you start specifying that the forward segments are going to form a head.?So you get cephalization. You can see that it's putting out gills and legs on most of its segments--but it's kind of stopping to do that on its back segments--and it's developed a hard exoskeleton. So this is steps on the way to becoming an arthropod.?It's just an intermediate form between a worm and an arthropod.?

We're currently in the middle of a big anthropogenic extinction crisis, but it appears like this isn't something that the planet hasn't experienced before. Geological processes have caused many extinctions of entire communities, wiped them completely off the face of the earth, and life has re-generated new ones again and again and again and again.?

Now what about stasis? What about the fact that the Coelacanth that you catch off the Comoro Islands today, looks almost exactly like the Coelacanth that's in the fossil record from 360 million years ago? What about the fact that the Onychophorans that you collect in Australia today are practically indistinguishable from the ones that you see in the Burgess Shale 505 million years ago? Why is there stasis??

• So stasis basically describes a long period with no morphological change. There's no apparent response to selection. Evolution doesn't appear to be going on. And it is puzzling, because we know that every nucleotide sequence undergoes mutations.

• There is no way that you can stop the production of genetic diversity in these organisms. So for 350 million years Coelacanths don't change, but probably every nucleotide in their genome has mutated, over that period of time. So there's been opportunity for change, but they have not changed.

• The examples of this include club mosses and liverworts, lungfish, Coelacanths, the priapulids and phoronids. You saw the priapulids--I pointed them out in the Cambrian- in the Burgess Shale shot--tuataras currently still existing on an island off New Zealand, and onychophorans.

And here are two possible explanations for stasis.?

• One is basically a selectionist explanation for stasis. It says that most of the things that we're talking about have some method where either a larva or a seed can find the environment in which the adult will do well. And so there is a selection of an environment?early in life, and that actually then selects the selection pressures that will operate on the adults. We see the adults, we don't see the larvae.

• The reason things stay the same is that young life history stages find the environment in which adult selection will take place, and adult selection is stabilizing. Intermediate values are selected for. Things don't change.

• On the other hand, there's a contrasting hypothesis, which is an internalist explanation. Basically it is that tradeoffs are creating the stabilizing selection--that's one possibility--so that instead of having an ecological explanation for why there's a long period of stabilizing selection, we have an internal physiological or developmental explanation of why selection has been stabilizing.

• If those things are laid down early, both in evolution and then in development, occur early in development, there's kind of an embedding. That means things have been in place that can't be changed without destroying normal development.?

The other major take-home message is--that I've already signaled as Cope's Law. And again, there are two options here.?

• We see bigger things when?there's just a neutral evolution. Adaptive radiations have been creating little things and big things. But there was more room on the upper end than there was on the lower end; therefore even though it's been random, we see an accumulation of larger things?because the upper limits are far away.

• The lower limit on body size is always nearby; it's one cell, you don't get smaller than one cell. But the upper limit appears to be redwood trees and blue whales, and at least at the outset that's pretty far away. That's up at about 100 meters, for redwood trees, and about 30 meters for blue whales. So that's one possibility.

• The other is that the reason that things got bigger is co-evolutionary. Co-evolution is shaping prey to escape and predators to kill, and prey can escape predators by getting too big to eat, and predators can kill big prey by getting bigger than they are. So this would be an adaptive life history hypothesis, saying that Cope's law results from a co-evolutionary arms race between predator and prey.?

And we don't really yet have a powerful method for disentangling these two effects. And I think if you look at their logic, you can see that they're not mutually exclusive; they can both be going on at the same time.?

So what does the fossil record tell us? It shows us a lot of stuff that we couldn't see at shorter time scales. We see a lot more detail in the recent than in the distant past. It looks like mass extinctions may open up ecological space, for the radiation of surviving groups. So it may be that you need an extinction before you can have a big radiation.?

Most things start small and get big. And there's a lot of stuff that's not on the planet at all anymore; there are no surviving descendants. So the fossil record has a take-home point, that's actually a puzzle that can be attacked experimentally, in part by people doing evolutionary developmental biology and phylogenetics, and that is, why is there stasis? It's common, and we don't have an explanation for it.?