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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.

[An image of Ross MacPhee’s book, End of the Megafauna appears onscreen, and illustrations of now-extinct animals from the book cycle next to it.]

MACPHEE: 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. So what has been lost?

[An illustration of a Giant Sloth Lemur appears onscreen with it’s scientific name Archaeoindris fontoynonti.]

MACPHEE: 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.

[An illustration of a Giant Elephant Bird appears onscreen with it’s scientific name Aepyornis maximus.]

MACPHEE: This is an elephant bird that weighed in at about 500 to 800 pounds, so three to five ostrich-worth in its size.

[An illustration of a Woolly Rhino appears onscreen with it’s scientific name Coelodonta antiquitatis.]

MACPHEE: This splendid animal, woolly rhino, which disappeared about 11,000 years ago.

[An illustration of a Giant Goanna chasing a wallaby appears onscreen with it’s scientific name Varanus priscus.]

MACPHEE: 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.

[A black-and-white cartoon map of the world appears, with drawings of various megafaunal animals placed over their geographic homes–some of which are filled in with gray.]

MACPHEE: 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.

[An illustration of a South American landscape circa 11-12,000 years ago, with various animals, appears.]

MACPHEE: 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.

[An illustration of a Madagascan landscape circa about 1,000 years ago, with various animals, appears.]

MACPHEE: And then there’s Madagascar. 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.

[On-screen text reads “How many near-time extinctions have occurred? Mammals–about 350. Birds–about 1,000. Reptiles–about 100. Amphibians–????”]

MACPHEE: 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.

[A cartoon of cavemen and cavewomen hunting various large animals with spears appears on screen.]

MACPHEE: This is one that would implicate people, which my colleagues lovingly call overkill. Using this form of identification,…

[An cartoon of a woolly mammoth confusedly looking at a dried up river appears.]

MACPHEE: …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…

[A cartoon of a meteor hitting the ground with some mammoths lying on their back appears.]

MACPHEE: 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.

[A map of the world appears, marked with climate zones (tropics, semi tropics, temperate regions, polar regions).]

MACPHEE: 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.

[A map of the world’s climate regions as they were 25,000 years ago appears.]

MACPHEE: The biggest difference has to do with the ice. Twenty-five thousand years ago, where we are standing right now…

[An arrow appears, indicating the area that is now New York City.]

MACPHEE: …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 zonation, 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 refugial 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.

[An illustration of a landscape in Rancho La Brea, California, circa 11 to 12,000 years ago with various animals appears.]

MACPHEE: Here for example, in this painting what we are seeing is a scene in Rancho La Brea. 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.

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.

[A photograph of Paul Martin with the dates 1928-2010 appears.]

MACPHEE: 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.

This is very important. They were not globally synchronous. 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 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. 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.

[A map of the world, labelled “The Sapiens Diaspora” appears, with arrows indicating the movement of homo sapiens from Africa to Europe and Eurasia, Eurasia to North and South America, and then onto the islands of the South Pacific.]

MACPHEE: 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 years ago.

Now, let’s couple the diaspora with the extinctions.

[The map is overlaid with points indicating where major extinctions occurred.]

MACPHEE: 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.

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.

So, there are 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.

[The map showing where extinctions occurred now has circles drawn around Africa and Asia, where there are no indications of extinction events.]

MACPHEE: 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 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.

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 then of course, the extinctions went to completion because there was so much overhunting. The end result was these losses.

Let’s go to overgrill. Overgrill — I don’t think I have to tell you a lot about this particular scenario.

[An artist’s interpretation of an impactor crashing into the ocean, with pterodactyls flying nearby, appears on screen with the caption “66 million years ago, Chicxulub Impactor”.]

MACPHEE: Sixty-six million years ago, an impactor from outer space hit, as we now know, somewhere off the Yucatan Peninsula,…

[A cartoon map of Central America appears, indicating the location of the Yucatan Peninsula in the southeast of Mexico.]

MACPHEE: …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 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. Like shocked quartz grains,…

[Two microscope images of shocked quartz grains, which are small mineral fragments that appear to have scratches on them, appear.]

MACPHEE: …which are all about the strength of the impact. Something peculiar to these Pleistocene examples of this particular impactor is something called black mats…

[Images of large rock formations with bands of darkened rock that almost appear scorched, appear on screen.]

MACPHEE: …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?

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.

[A cartoon map of the world appears with icons of extinct megafauna colored in either red, to indicate human-caused extinction; blue, to indicate climate-caused extinction; or purple, to indicate insufficient data, appear. A mammoth icon and a rhinoceros icon are colored in both red and blue, indicating multiple causes of extinction.]

MACPHEE: 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.

[A photograph of Sudan, the last male northern white rhino, grazing at the Ol Pejeta Conservancy in Kenya, appears with the caption “Died March 2018.”]

MACPHEE: 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.

[APPLAUSE]

[Credits appear.]