Podcast: SciCafe—End of the Megafauna with Ross MacPhee

ROSS MACPHEE (Curator, Division of Vertebrate Zoology, American Museum of Natural History): Thank you. It’s nice to see what a crowd this is. I did just recently publish this book. I think you should all at least look at it, and otherwise enjoy it, because it’s going to answer all your questions.

What happened to the world’s megafauna? Is the extinction a thing, and should I be worried about it? Did I, Ross MacPhee, ever eat a mammoth steak on the Siberian Tundra? The answers to which are yes, yes, no. In a nutshell, End of the Megafauna, the book, is about a lost world. One not that different from ours today, but sadly diminished; diminished in the sense that all of the largest, fiercest, hugest, as the subtitle goes, animals, with few exceptions, have disappeared.

How it came to be so is what I want to talk about tonight. We are going to be talking about theories, and we are going to be talking about evidence, because that is what this institution is all about. What has been lost? One of the things you are looking at right now is a gorilla-sized lemur from the Island of Madagascar. This too, was a Madagascan vertebrate. This is an elephant bird that weighed in at about 500 to 800 pounds, so three to five ostrich-worth in its size.

This splendid animal, woolly rhino, which disappeared about 11,000 years ago. And finally, this relative of the Komodo dragon, which is the world’s largest living lizard. This guy was 30 feet long, lived in Australia up until about 40,000 years ago. You’re not the only one that’s terrified, so is this wallaby, who is about to be a meal.

We still, of course, have lots of megafauna left. We have elephants, ostriches, crocodiles, all of these animals that are 100 pounds or better, which is the body size that we use to define megafauna. What you are looking at now is an illustration of mammals of two kinds. The ones in grey are the super megafauna, the really big guys that come in at 2,000 pounds or better. You see that they are restricted to elephants and rhinos that live in South Asia and Africa. Everything else you see, which includes some smaller animals as well, have completely disappeared.

You notice, for example, that there aren’t any super megavores that are left in the new world, whereas there were just a few thousand years ago. What does this mean? It obviously means that there has been a lot of losses. What I want to bring forward to you so you appreciate the level of loss, is that this is a world-wide phenomenon. It’s not just the new world, it’s not just Europe; it’s practically every island, habitable island, on the face of the planet, that has lost a very substantial part of its fauna in the last 50,000 years. Fifty thousand years is what we now call near time, as opposed to deep time that real paleontologists deal with.

This is South America shortly before the extinctions that occurred there. Giant ground sloths, giant armadillos, other weird beasts; the only living lineage is represented by this canid, this dog-like animal that you see here, which still survives on the continent.

Madagascar has lost practically everything of size that used to live there as recently as 1,000 years ago. All of the big lemurs, pygmy hippos, native crocodiles, elephant birds, giant tortoises, they are all gone. Everybody here, with the exception of this snake bird, anhinga, which still lives in Africa and Europe and elsewhere, it is the only survivor that you are seeing in this particular painting.

Here is the body count. This is based on a number of assumptions, because we don’t necessarily have good evidence for the totality of these losses. Within a ball park kind of frame, it is probably fairly accurate. Somewhere around 300 to 350 mammal species. Many more birds, because bird extinctions on islands, particularly in the South Pacific, have been prodigious indeed. Reptiles, we’re not really sure. I’m saying 100; it could be much more, it could be much less. Amphibians, we have not idea whatsoever how many have been lost in near time.

This raises the question, what the heck happened? There are many ideas, and ideas is what we work with. This is our currency in science — developing ideas, testing them, and seeing what comes out in the wash as the ideas that are best supported by the evidence.

In the case of near time extinctions there are a lot of ideas, some of which we will be able to talk about tonight, but I’m going to group them into three units. First of all, there is ones that are concerned with climate, climate change, because in the period that we are talking about 50,000 years ago, there was an enormous amount of change on the planet, because that was the last ice age that we have now come out into and are now in a warming period, a very warming period.

Then there is a number of arguments that would suggest that people were, if not universally responsible, at least responsible for a large number of these losses. We are going to take a close look at those as well. Then there is other ideas. Some of these are half-baked, wacko, but the point is they have been proposed because the other main theories, which would be climate change and people, have deficits; they have problems with the argument.

That is what I want to bring out tonight, so that when you walk away from this it’s not, “Oh, MacPhee, he just can’t make up his mind.” Instead, it is that this is the way the process of science actually works. You accept nothing. You look at the evidence, and after much grueling effort you decide what you are going to support. And you also look at ideas that are on the fringes because from time to time those are the ones that are correct. Just ask Albert Einstein.

We’re going to use some of Patricia Wynne’s cartoons to illustrate these major ideas. This is one that would implicate people, which my colleagues lovingly call overkill. Using this form of identification, we are going to go to climate change, and we are going to call that overchill. Then the final one we are going to look at is about an impactor that might have hit the planet 12,900 years ago. What do you think we’re going to call it? Overgrill, of course. Thank you, thank you.

Because we don’t have a clear idea at all what was really responsible, we’re looking back; we don’t have a time machine, all we’ve got are the kinds of evidence we can collect in the form of bones, now with biomolecules to a certain extent, dating through radiocarbon. This is what we have to conjure with, and put together stories that are worth testing.

Let’s begin with overchill, and let’s talk about the facts. In grade school we probably all learned about ecological zonation. Ecological zonation being, in the simplest form, that we’ve got a tropical part of the world around the equator, we go into the semi-tropics, temperate regions, and then high latitude polar. This is what we think of as normal, because this is what we’re used to. But you only have to go back 20,000 years to see a very different world indeed.

The biggest difference has to do with the ice. Twenty-five thousand years ago, where we are standing right now was under ice, ice that originated in the northern part of Canada and spread out from there as the illustration suggests, all across Canada into the northern tier of the U.S. Smaller, but nevertheless still very large ice caps, existed in Eurasia as well. Among many other things, just the size of these objects made a difference. There was so much fresh water that was locked up in them, that sea level dropped about 120 meters, well over 300 feet. There was so much water locked up and it was so cold that evaporation over the world ocean was lessened, which means there was less water everywhere. That had a prodigious effect on plant life.

For example, here, if you combine the white and the blue areas, the blue area being step of one form or another, you can see that the whole northern part of the planet was in very, very cold or semi-cold conditions with reduced plant life, and so forth. Everything else, all of this [tonation], was packed much more towards the equator; unlike today, where you’ve got temperate regions going well up into the northern part of our continent.

One of the things that happened was that the rainforest, having less precipitation, generally cooler conditions, reduced down to refugia areas. If you have in your mind’s eye an idea of how big the rainforest is in the Democratic Republic of Congo, for example, you can see here that it is a fraction of that. Similarly, with Amazonia. Amazonia was cut up into bits with grassland intervening, which is a very hard thing to imagine today, as the result of climate at the time.

What it also meant was that grasslands could expand dramatically since they are dry-adapted, and the mid-latitude deserts. So yes, this world looks a lot like the world today, but in detail it wasn’t. This was the world of the megafauna. Here for example, in this painting what we are seeing is a scene in Rancholabrean. Everything here is dead, it’s all disappeared including sloths, camels.

Does everyone know that camels originated in North America? You don’t have to like camels, I’m not asking that you like camels, but they’re yours, they’re your own. You should respect them. Horses, similarly, originated here. Not true of ground sloths. You see a couple of them here. They originated in South America, but they had a successful run in the north as well.

So the climate change people don’t have a great answer to how climate change by itself could have produced these effects. The particular effect that I’m talking about in respect of North America was the collapse of this fauna, some of which is illustrated here, in a very short period, maybe no longer than 400 years between, roughly speaking, 11,000 and 12,000 years ago.

The big ice age which you’d think would have been responsible for these extinctions, was over and done with by about 18,000. From 18,000 forward, we were into a warming period and thinks were not exactly like today, but they were getting warmer. So how can it be that the most dramatic effect of the recent period produced no extinctions, or very few?

Another aspect of this is that there was more than one ice age. In fact, there were 22, that we know of, advances and retreats of the ice in the last 2.6 million years, which is the Pleistocene and the Holocene. During this period, most of the species that I’m going to be talking about were already in existence. They’d already evolved. Yet, they weathered those particular ups and downs of climate. They may well have suffered in respect of range size, they may well have suffered in respect of the kinds of food they could eat, but they made it through until 11,000 to 12,000 years ago. Who knows what happened?

Well, let me tell you. This man, Paul Martin, thought he knew. Paul Martin was a good friend of mine, was a professor of geosciences at the University of Arizona for many years, and he was really the one who developed and elaborated the theory that I am now going to talk about — overkill. This is how he approached the question.

He developed a series of three features, which I am going to briefly go over, that seem to be true of this entire period of near time. These are structural features of what was going on, and in each case there is verification from various bodies of evidence suggesting that Paul was correct in isolating them as critical features.

The first one there is no real disagreement about. Where the extinctions occurred, there were a lot of losses. By no disagreement, I mean people supporting climate change or other theories also agree with this. And that large mammals were affected disproportionately. There might have been more bird extinctions, per se, but the mammals, in terms of diversity, were more affected. This is very important. They were not globally synchronous.

I’m going to go over this in some detail, so I’m just leaving this as a hanging thought at this point. You’d think if there was one serious effect that was global in extent, it would have affected all of these species that disappeared at more or less the same time. Yes, there could have been survival in some places, greater losses in other places, but you’d expect to see a general envelope of synchronicity. That is what we don’t see, and we know that from carbon 14 dating, which was Paul’s biggest contribution, by pointing out that you could date, to a very large degree, when these extinctions occurred.

Another thing that’s very important and not to be minimized at all, is that when people spread across the planet from Africa, which occurred over a couple of hundred thousand years, it seems that whenever they got to a new place where they hadn’t been previously resident, the next thing that happened was a faunal collapse of serious magnitude on most of the continents, and absolutely devastating on islands.

What you have to picture is that as people spread across the planet, every time they showed up in a new place, something bad happened. It was bad to be big all over the planet during near time. We call this first biological contact — the idea that when people came to a new place and encountered the biota that lived there at the time, it was new on both sides.

Let’s look at what we’re calling the diaspora here. This is a very simplified version, of course, of what happened as homo sapiens spread across the planet. Point number one — we originated in Africa; homo sapiens is African by origin. That is our lineage. The dating now is that we were already present at least 350,000 years ago in Northern Africa and at some time shortly subsequent to that, we got into Eurasia. Thereafter, it’s a question of spreading wherever we could possibly go. By 65,000 years ago, so the current evidence is, we were probably in Australia. We were in northernmost Siberia by 45,000 years ago. These are minimum estimates.

We got to the new world somewhere after the close of the Last Glacial Maximum. This is a very controversial area, but let’s just say 16,000 to 18,000 years ago people were in the new world for the first time. Finally, people spread throughout and into the last habitable places on earth, which would be the temperate and equatorial islands, wherever they lay, with the South Pacific being the last of them. There were people, for the first time probably, in New Zealand about 800,000 years ago. 

Let’s couple the diaspora with the extinctions. The first major losses, and by major I mean lots of species within an apparently short period of time, so in that sense at least temporally connected. This was in Australia about 40,000 years ago as best as we can estimate at the moment. Remember, people seem to have got there about 65,000 years ago, so there is a gap, but nevertheless the losses occurred subsequent to the first human appearance, at least for the Pleistocene.

Thereafter, attention goes through the northern tier of Eurasia, down into North America because people were able to either cross the Bering land bridge. I shouldn’t say people, that begs the question. Animals were lost around 11,000 to 12,000 years ago in all the places where the yellow explosions are. So across the northern tier of Asia, down into all of North America and all of South America for that matter.

Then it’s the islands in the Mediterranean around 10,000 years ago. The islands forming the West Indies, perhaps 6,000 years ago. Madagascar, about 1,000 years ago. And then all of these last places on earth where people could make a living, the South Pacific, with the very last of them occurring perhaps only a couple of hundred years before Captain Cook ended up in New Zealand.

It is this coupling of the first appearance of humans and the subsequent faunal collapse that gives particular strength to the overkill argument. It seems we got there, we did something hideous, which is not like us at all, right? And the animals died in droves. That’s fine, we’ll get to the overkill argument and that kind of detailing in just a section…session…second. If it was alcohol, I would be more fluent.

So, there is puzzles connected with this. Here is mystery number one — you saw the pattern of loss. Northern Eurasia, continents of the new world, all of these islands, so on and so forth. But there were some places where coupled losses, lots of them in a short time period, didn’t occur. Those are Africa and South Asia. What do you do with this? This is a major puzzle because, as I said, humans originated in Africa, so we go back and the mammals that we dealt with through this long history were all there at the time. How could it be that there weren’t all of these losses in those places, yet there were everywhere else?

Well, Paul turned this inconvenient truth, if I may call it that, into a positive for his argument by pointing out that since people arose there and lived there for a very long period of time, they were known to the prey species. There was sort of arms race that went on. As we got better at making tools, they got better at avoiding predators. So, over time, the balance was that yes, hunting was possible, but this destruction on unbelievable scales like you saw in North and South America, did not happen. He made the same argument for the south. End result, no faunal Armageddon in Africa and South Asia, but destruction everywhere else.

Okay, so we’ll accept that. Another problem — in North America there were about 70 species that disappeared between 11,000 and 12,000 years ago according to the paleontology.  Not all of those were large, more than 100 pounds, but probably around three-quarters were. So you’re talking about a large number of large animals. You’d think that if there was a mass slaughter, that we’d have good evidence of it, that there would be all kinds of kill sights, windrows of bones, and evidence of butchery, and all of this kind of thing if people were killing on this kind of scale.

In fact, we don’t see that. What we see instead is the very occasional evidence of hunting. I’m talking just hunting, not overhunting. This mastodon, for example, from Washington State, we know this guy was hunted because that’s a spear point...stone point in one of its vertebrae. I think any reasonable person would accept this as a kill. But it’s a singleton. It’s not like there is lots and lots of individuals associated with it.

Over the continent, there is about 15 sites that have either mastodon or mammoth kills something like this, where there is objective evidence of human presence. There is one site in North America of horse hunting. There is one site of camel hunting. All the rest of the megafauna, all the big sloths, and the giant rodents, and the sabretooth cats and whatever you want, there is no evidence of this sort.

So the question comes, where are the bodies? If people were responsible at this level, shouldn’t we expect to see more? Paul’s argument, not a very strong rejoinder I must say, was that we can’t really expect that because this is far back in the past, 10,000 years or more. And the absence of evidence, you’ve all heard this, absence of evidence is not evidence of absence.

You know, try that out with your significant other when you come home late one night. See where that gets you. It doesn’t cut the mustard. It’s an explanation that we don’t know what to do with, or it’s a set of facts that we don’t know what to do with. You’d expect that under any of these circumstances, that there should be lots of bodies, but there isn’t. That’s a conundrum.

Now, let’s think about this. You are recently arrived in North America. You’ve got across the Bering land bridge, you’re seeing all of these big mammals that nobody has ever hunted. They look at you and they see this awkward looking, featherless biped, doesn’t seem to know what he’s all about. There’s no big canines, there is no horns or other ornamentation, just running around with pointy sticks. What’s that all about?

The idea here, which is borrowed from ecology, is that you can have the phenomenon called behavioral naiveté — that unless you have been, for example, predated upon very heavily, you as an animal are not going to recognize a predator, particularly a novel, weird looking predator like a hominin. So you are just going to ignore it. But meanwhile, those humans are seeing all the meat on the hoof and thinking, “All right, party time.”

Paul’s argument was that when these first inhabitants came over, if they weren’t already big game hunters, they quickly became such because what did they want? They want what humans want in all circumstances, which is to control all resources and to make as many copies of themselves as possible. The idea was that there was a massive population expansion right at this period when humans were first moving into the Americas. Now you had food security, you had all of these guys that you could easily hunt, and you could go forward with that because it seemed to be an endless resource. 

I’ve got the fool from a tarot deck to give you the idea of what all of this is about. The view here is that the animals were blithely ignorant of humans, unlike their African relatives which had figured out hominins a long time ago. Until they sorted it out, if they were able to sort it out before going extinct, the idea was that it was fat city for humans. Then eventually, of course, the extinctions went to completion because there was so much overhunting. The end result was these losses.

Overgrill — I don’t think I have to tell you a lot about this particular scenario. If you are regular visitors to the museum, you know what this is all about. Sixty-six million years ago, an impactor from outer space hit, as we now know, somewhere off the Yucatan Peninsula, and whammo…end of story. Whatever happened…nuclear winter, global wildfire, you name it. It took out around 75%, so we estimate, of species then living. They weren’t all dinosaurs. It went all the way down to planktonic forams living in the ocean surface.

The parallel argument for us is that there was an impactor. The people who developed this argument coyly suggested that it came down at about 12,900 years ago and wherever it hit, we’ll get to that point in a minute, wherever it hit it did the same sort of thing, but probably on a reduced scale, as compared to the Chicxulub impactor 66 million years ago.

How do we know this? There is evidence. Just like the iridium anomaly gives us an idea that it must have been an extraterrestrial visitor because we don’t have amplified amounts of iridium on this planet, it just didn’t happen. These asteroids originating elsewhere billions of years ago have elevated rare earths of all sorts, so when they smack down you get these rare earth anomalies. That means something happened.

There is also other proxies or other indicators like shocked quartz strains which are all about the strength of the impact. Something peculiar to these Pleistocene examples of this particular impactor is something called black mats which are found in the southwestern part of this country, which the supporters of overgrill suggest are exemplary of a very widespread fire initiated by the impactor.

I think it is distinctly possible that there was an impactor at this time. The fundamental question for us, however, is did it do the job? Did it do any of the jobs in respect of the losses that we are talking about? Well, here is something to think about. This graph, the one in red, is estimates of temperature over roughly the last 18,000 years. When you are low down in the graph it is cold, and then temperature elevates. When you get up to the modern period at zero, you can see temperatures quite elevated.

You’re talking about a 20-degree difference, which is very substantial in Celsius. The other thing to notice is that in this interval right here, right around 12,900 years ago, there was a very dramatic drop in temperature. We had been in a basically warming phase coming out of the Last Glacial Maximum, it’s getting warmer, and then hit the skids. Within a period of something like 50 to 100 years we went down dramatically and stayed there for the better part of 1,000 years and then very quickly climbed out of it.

This is called the Younger Dryas Interval. So I don’t get the question, yes, there is an older dryas, a geriatric version, but I’m not going to talk about that because it’s not relevant to the problem. Now, what does this mean? How can you get that kind of change so dramatically in such a short period of time? People have conjured with this for a very long period, and nobody has come up with a very satisfactory answer, except for this one. It’s satisfactory to me, which is why I’m going to tell you about it.

We’re in the warming period, it’s post-glacial times. What that means, however, is that the ice caps retreated relatively slowly even though they were melting. The melt waters, in the case that you are looking at here in central Canada, gathered behind one of these big ice sheets. Because it was basically a basin, they had nowhere to go so the water just kept getting deeper and deeper. This is called Glacial Lake Agassiz and it is in the position of Lake Winnipeg at present.

Over time, the amount of water in Glacial Lake Agassiz matched that in the Great Lakes nowadays. Just think of that in terms of the volume. At some point, we think, there was a catastrophic unloading of this water. The question is, how did that happen? One of the ways in which it could have happened is that the ice dam gave way. Remember, ice floats, so at some point the basal pressure on the ice cap was such from all the water that it just forced it up and you had an enormous release of water, all at the same time.

That’s okay, but there still has to be a triggering event, right? The supporters of overgrill said, “Why couldn’t it be our impactor?” Well, it could, but the trouble is there is no crater. So you are asking me to believe there was this thing from outer space, hit the planet, caused Glacial Lake Agassiz to dump all of its water, but you can’t even tell me where on the earth’s surface it impacted?

If you follow Science News, just a couple of weeks ago in late November there was an argument from the Danish Museum of Natural History, some of my colleagues in Copenhagen who have worked in northern Greenland for many years. They believe they have found an impactor. Structurally, it absolutely looks like that. It’s called the Hiawatha for reasons unknown to me, but it’s right there up in the northernmost part of Greenland.

What their suggestion is, is that this could be the impactor that I’ve been talking about, but it is very difficult, under these circumstances, because Hiawatha is basically under an ice sheet, to date it. The way the press release read is that this could be 12,900 years ago, it could also be 3 million years ago. There is a lot of latitude here, and that’s the trouble because what we’re interested in is its relationship to extinction.

There are other problems. That little diagram on the side is this conveyor belt of thermohaline circulation, which I’m absolutely not going to go into detail about. What it is responsible for is equilibrating temperatures across the northern hemisphere in particular, thanks to the Gulf Stream being carried up towards the north. Places like Britain and Iceland and so forth, despite their high latitude, enjoy a more equable climate than they would otherwise have. It’s a heat transport mechanism.

In this scenario, this is meant to be Hiawatha hitting in northern Greenland, the idea is that whatever it did in terms of catastrophe went south through North America, into Eurasia, and over into East Asia. That correlates well; if it did hit here 12,900 years ago, it correlates very well with the losses that we know about in these regions. 

But here is the problem. As I’ve already mentioned, there was also a big extinction at roughly the same time, within a few hundred years, in South America. South America lost just about as many mammals as we did up here. If we are arguing that it was disruption of the conveyor belt, which is the favorite argument, that there was this massive release of fresh water that shut down thermohaline circulation. It took a while, like 1,000 years to start working properly again. Then you’d expect to see these knock-on effects all the way down, but you don’t.

You don’t see losses at this time in the Great Antilles and the West Indies, for example, where I have worked. In particular, you do see it in latitudinally equivalent parts of Africa, which you would think would have happened because the conveyor belt works all the way through the Atlantic Ocean, this part of it. Those are severe impediments.

You won’t be surprised when I say that we don’t know very much about exactly what happened. But to give you one note of optimism, thanks to lots of thinking and effort, we do have a good idea why large mammals were hit in particular. This is really easy to relate with, because we are large mammals; at least I am. Here is how it works — large mammals have very similar life histories, as we call them, for certain features. One is that gestational periods are long. We’re nine months, but if you go to the living elephants, it is anywhere from 22 to 24 months. Typically, there is also, relative to smaller species, a longer maturation. Again to point to elephants, it’s something like 10 or 11 years before they are sexually mature in the case of African elephants; 12 to 14 in the case of Asian elephants.

Yes, we live long lives. That’s another part of being big. That’s one of our, I guess, positives in life history. These other features are ones that mean that if our populations get hit very hard in certain parts of the life cycle, certain parts of the demography, then it takes a long time to come back. For all of these arguments, what we really should have in mind as to why it was bad to be big is not the details of what specific trigger mechanism took them out, but because they suffer or they have as their native aspect that there are these risks you take as a result of being big, that smaller animals don’t.

One other thing to mention is that most births for larger animals are singletons. Twins rarely, anything larger than that is unusual. You can imagine if you get hit very hard, what it is going to take to recover from that. Whether it was climate change, overkill, an impactor, it doesn’t matter.

I want to leave you with another kind of idea, which is since we don’t know the whole story and since there are these arguments against any of the theories that I have developed here, maybe the most serviceable idea is that it was some combination of the likeliest events, which is what this particular figure is trying to show. If you can’t read it very well, the ones that are in red, the suggestion is that it was human depredation, overkill. The ones in blue or partial blue suggest that there was predominant to slight climatic effects, and in other instances, which applies to South America and Africa in this case, it is because we really, really don’t know as opposed to just being a little bit I don’t know.

I kind of like this because at this point in our understanding of these megafauna extinctions, there is no point in being a very strong advocate of any one of these solutions. They all might have played a role. The point is, and the reason why this is still a great natural history puzzle well worth spending our time on, is because it has certain implications. This is the implication that I want to leave you with.

Why should we be particularly worried about the surviving megafauna on the planet? It’s because they have these same life history characters that I was just talking about. Here you’re looking at the last male northern white rhino that died out earlier this year. Fortunately in this case, I suppose fortunately is the right word, semen was collected from him and in future there is going to be an effort to see whether you will get viable hybrids, as you probably will, with southern white rhinos.

You can see the handwriting on the wall. As we continue to despoil the planet, taking resources wherever we want to take them, doing what we please, then inevitably animals like this are the ones that are going to suffer most. The end of the megafauna that I’m talking about is not necessarily the end of the end, and I want you to bear that in mind. Thank you.

Let’s have a question.

AUDIENCE QUESTION: Thank you very much for a wonderful, stimulating talk. I love the way that you asked questions and questioned the answers. I understand how overhunting might have resulted in the loss of the large mammals, but what happened with the birds? Were we such prolific hunters that we went up into the mountains and killed the birds, or up in the trees? How did the birds die?

MACPHEE: Okay, good question. How did the birds die in all of this? Moas, which were the dominant avifauna of New Zealand, were very large birds indeed. Not as large as elephant birds in Madagascar, but nevertheless very sizable. They did not fly, and in fact that is the pattern for most island birds. You can think of it this way — it takes a lot of effort to get out there to the island. Once you are there, it is, “Phew, no predators, I don’t have to worry anymore. So I can bulk up and eventually lose powered flight.”

Most of these 1,000 birds that I mentioned early in the talk that were victims of these extinctions, were flightless rails, rail being a kind of bird that populated most of the islands in the South Pacific. They all had flighted antecedents, but they very quickly lost the capacity to fly. Here’s the story — from an island that is just off Australia, which gives you kind of a dimension to this, that part of the world was first seen, not by Polynesians, but instead by explorers, late 18^th century.

They landed there, and there was a pigeon. This pigeon had this behavioral naiveté that I am talking about. So what the sailors did when they wanted a few squabs was go up to a tree and just grab one, and then break its legs. Why would they do that? Because then it would cry hideously, and that would bring all of its relatives, other birds, around to see what was happening. Because they still didn’t get it, the sailors could just knock them out of the trees as well. This is part of this vanished world that is hard for us to understand, because wild animals nowadays don’t react to us like that; they know the score. But it seems before then, when there was behavioral naiveté all over the planet, this is the kind of thing that happened.

AUDIENCE QUESTION: Thank you for a great talk. Going back to the idea of an arms race leading to the preservation of megafauna in Africa and South East Asia, has there been any attempts to explain the fact that there might be evidence for pre-human species like homo heidelbergensis or homo erectus actually engaging in hunting outside of Africa in Europe or Asia?

MACPHEE: Yes, there is a very recent paper on that topic, but I’ll wind around to that. First of all, hominins have always hunted. I accept that, that’s part of the reason why you make stone tools, so you can gather meat resources as well as plants. The recent paper I am talking about was an effort to establish whether or not there was any connection between any of these losses in places like Africa and earlier hominins. Hominins being the main tribe, members of the genus homo, including us as homo sapiens, but including other antecedent species like heidelbergensis. Neanderthals definitely hunted and they had the technology to do it.

What was found, and I believe this to be the case, is that these coordinated, very large-scale extinctions, didn’t happen. At whatever amount they hunted, it didn’t so deplete species that they actually collapsed. To go back to the overkill argument, there is something about our species arriving first in a place where we’ve never previously been, and turning it into a grocery store for ourselves.

Whether that means that overkill is the explanation, or something else, because there are certainly other arguments that I haven’t explored with you tonight, we don’t know. But this cloven hoof print of humans like us, sapiens, is all over the plant. With earlier species, we don’t seem to have the same sort of thing, earlier hominin species.

AUDIENCE QUESTION: First I want to thank you for a very illuminating presentation. That raises a lot of questions. What comes to my mind is we learned how, when the Europeans came to North America, we infected with the Native Americans with diseases that they had never been exposed to before. Since the animals which were living with the hominids in Africa had been exposed to our diseases, could it be that when the hominids left Africa, that they exposed the animals to diseases that they had never been exposed to before?

MACPHEE: Thank you, ma’am, that is a brilliant insight. It just happens to be one that I thought I had about 20 years ago, and published as the hyper-disease…this is New York, forgive me…the hyper-disease hypothesis, and it was exactly that. Very briefly, I didn’t talk about it tonight because it didn’t make the cut. It was just not good enough. Fireballs, currently, are more interesting than pathogens. Here’s how it works, and it works in precisely the way that the questioner asked.

As a result of our exposure, and within our lineage over very long periods of time, we have native immunity or near-immunity to a lot of diseases. If we do get them, they tend not to follow a very serious course. That’s because the disease pool our ancestors were involved in. When we go from a place, from our disease pool, into another place where they’ve never experienced or met up with those same sorts of pathogens, then it can be very bad indeed. 

What happened with Native Americans, when the conquistadors and later came over, it wasn’t by force of arms that they destroyed Montezuma and the Aztecs. Instead, it was diseases like measles. In European populations, our antecedents, this is a disease that is a bother when you are a child, it’s not a killer. But it can be, it all depends on your constitution and your genetic inheritance. 

What the hyper-disease argument was, was an effort to explain how humans could have first come, say to North America, but not killed everything in sight, but still brought over something, did something, that resulted in these extinctions. The idea was that maybe there were these diseases that became emerging diseases, like Ebola in humans right now, or HIV in humans, that the native species in North America had no immunity to. So they died in droves. There are examples of that. I don’t want to…you got me started, so this is your fault.

I would mention a particularly interesting, because I’m morbid, a very interesting case in Central Asia just two years ago with the Saiga, which is a kind of antelope. What happened, in a nutshell, was they were doing fine, the birthing season had just finished, and there were lots of calves around. Then suddenly, they started falling over with very severe fevers, very severe effects of all sorts, and died within 24 hours. Thousands of them within the course of a very short period. The veterinary pathologists didn’t know what was going on.

Was this a new virus? Did it come in from somewhere? No, it turns out that it was a bacteria that is ordinarily present in the respiratory tracts of ungulates where it does no harm. But somehow, for some reason which is still unclear, in these cases in this particular animal, it managed to enter the bloodstream. What these animals died from was sepsis, blood poisoning. If you or anybody you know has ever had sepsis, you know what a severe kind of infection it is. It’s very high mortality.

So, end point — could disease be an explanation for some of these extinctions? Possibly in some island context. We think we have evidence in one particular case, which I’m going to spare you the details of. But why not? There is nothing out there in nature that could bring a standing population from normal down to a fragment of what it was like serious, emerging, infectious diseases. It’s just another one of these things that is out there as an idea. Maybe some skinny graduate student some years from now, will be reporting from this stage, “Well, MacPhee didn’t have it completely wrong.” Which would be nice, I hope I’m there to see it.

AUDIENCE QUESTION: Thank you, Dr. MacPhee. You’ve spoken about the terrain animals, the large animals. I’m wondering about the whales and dolphins. Is there any correlation between the whales and dolphins and the terrain animals?

MACPHEE: Excellent question. There was no megafaunal extinction in the seas at this point, or at any point up until very recent times…within the Pleistocene, I mean. Now, here’s what I want you to think about. What Paul Martin was suggesting with overkill, was that the people doing it, this was their job. This is like 24 hours a day, “Oh, I’ve got to get up and kill me some mastodon, otherwise the family won’t have enough ton of mammoth or mastodon to eat.”

This is so unlikely. It’s so unlikely for very good, ethnographic reasons. What we know about band level organization, which is surely what it was in North American 18,000 years ago, is that the people were basically hunters and gatherers. They didn’t have stable agriculture or anything resembling it, they relied on what they could pick up in their round over the course of the year. There are still people who live like this.

They don’t have any kind of bureaucratic organization the way that we would understand it. There is no leaders. There is kin groups, there is people that you ordinarily associate with who are perhaps outside your kin group, in the daily round. You might have meetings now and again to exchange marriage partners or what have you, but that’s it. Once that job is over, they disassociate.

How could it be, 18,000 years ago, that you had bands of people coming in and suddenly saying, “Yeah, let’s get organized. Let’s take out those mammoths and ground sloths, and so on, because we can.” It just beggars the imagination that this would be possible. I’m saying all this because the way to think about this, this is sort of industrial level overkill. You are occupying your whole time with taking out animals. Remember, it all happened in the course of 400 years, so the argument goes.

So that is the most similar kind of exploitation we can think of in modern times? I would suggest there is two of them. Seals, particularly in the 18^th and 19^th century, and then whales right up to the moratorium, when prodigious numbers of these animals were collected for obvious reasons. What has happened? In the case of seals, there has been one verified extinction which happened back in the 1950s, not centuries ago, but you could pin it on over-exploitation.

Seals have come back, fur seals in particular, in such an enormous way that in places like south Georgia…when Captain Cook discovered south Georgia in the 1770s, he suggested that sealers should go down to Antarctica because there were so many animals, you’d never run out of that resource. That’s what they did, until they did run out of that resource.

Since then, since seal fur has gone out of fashion, the numbers have climbed hugely. That’s true for every other seal, with just a couple of possible exceptions among the monk seals. Despite the fact that we were hitting them very hard with modern technology, we did not seemingly cause a lot of extinctions.

The same with whales. Not going to go through all of the data, but the point is there has been a moratorium for now something like 40 years. With the exception of the Greenland right whale, every other whale, particularly those that were heavily hunted, are coming back.

That is a good thing, but it’s also something to think about from a scientific perspective. Despite the fact that we killed tens of thousands of these animals in a very short period of time…industrial level whaling was basically a 19^th and early 20^th century undertaking, we didn’t cause extinctions. Had there been no moratorium? I’m not arguing that point. It could well have gone downhill from there. But the point is that there was intervention. Whaling, except for a few rogue countries who will go unnamed, it stopped and the whales are coming back again.

Once more, when you think about overkill, what kind of effort would you have to have expended 18,000 years ago to literally kill everything there was. Even when you are thinking about life histories of some of these large mammals, you still have to take out a lot and make sure that they don’t come back again in some fashion. Which is ridiculous, because there is no way you could make the argument that early hunters in North America were interested in causing extinctions. They were in it for the meat.

Anything else?

AUDIENCE QUESTION: What percentage, aside from the cats, were actual carnivores? They seem to be all grass eating, leaf eating. But other carnivores?

MACPHEE: Sure. Somewhere along the line you are going to see something called the short-faced bear. What was the short-faced bear? It sounds so innocuous. If it’s got a short face, it can’t be much of a worry, right? Wrong. This is like a Kodiak on steroids. This was one of the largest carnivores in recent times. It had enormously elongated limbs, so it is a Kodiak on stilts. It came in maybe 2,000 pounds, maybe more. These were the largest carnivores in recent times in the Americas. There is one surviving lineage, Tremarctos, in South America. They have otherwise disappeared.

Dire wolves, relatives of the living wolf, disappeared. In addition to sabre tooth cats, there were other kinds of cats with big canines called Scimitar cats, which were part of the landscape. There was an American lion that got over the Bering land bridge sometime early in the Pleistocene. Made a living for a while but disappeared; very closely related to the African and Asian lions. There were large jaguars that have also disappeared.

So, what’s my point? My point is that you had a carnivore guild that was easily as large as anything that you see in Africa or southern Asia today, which has collapsed down to virtually nothing. Yes, we have wolves. Yes, we have coyotes. Yes, we have a couple of species of bears and we’ve got the puma. But that’s a fraction, and mostly a small fraction in body size, of what existed here until recently. It’s probably easy to understand. With all of their prey species going down, the large herbivores, what was left? They went down with everything else.

MODERATOR: Please join me in thanking Dr. MacPhee.