Image credit: J.T. Csotonyi [CC BY 2.5] (source)

Snapshots of Prehistory: A Vacation in the Geologic Past

A good friend of mine has kindly agreed to let me borrow her flying time machine for a week. I’ve never flown one of these before, but she has given me thorough instructions on how to operate the craft. My plan is to spend each of my seven vacation days visiting a different time period in geologic history. I’m looking forward to seeing what is familiar and what is unfamiliar in each time period I visit.

Day 1 — 100,000 years ago

For my first day of vacation, I’ve gone back 100,000 years in time. As I land my flying time machine somewhere in the southeastern part of North America, the landscape looks rather familiar. I see a lot of forest around here, and even from a distance I can recognize oaks, pines, maples, and so on. As I step out of the craft to examine these plants, I realize that almost all of them are familiar species, with only minimal differences from their modern equivalents. A period of 100,000 years simply isn’t a lot of time when it comes to the evolution of large plants and animals. However, smaller creatures with a faster lifecycle can evolve quite a bit in that same amount of time.

As I take a short stroll outside of my time machine, I realize that the weather seems a bit cool for a summer day, but not excessively so. The Ice Ages, which started about 2.8 million years ago, consisted of 17 glacial periods and 17 interglacial periods. During each glacial period, the ice sheets expanded — and during each interglacial period, the ice sheets retreated. I happen to be visiting at the tail end of the last previous interglacial period, when the climate was only slightly cooler than modern times. In other words, I have arrived just before the start of the 17th glacial period. This final glacial period lasted until 11,700 years ago, when an interglacial period called the Holocene began. All of the Ice Ages prior to the Holocene are lumped together into an epoch called the Pleistocene.

I see some squirrels scampering around, and I nearly step on a snake — which makes me curious as to what other animals I might find. I get back in my craft and begin to fly westward over the countryside, all the way across North America to the Pacific coast. Along the way, I count a surprising number of very large animals that I have never seen before — mastodons, mammoths, giant ground sloths, giant beavers, glyptodonts (which look a bit like giant armadillos), large camels, short-faced bears, dire wolves, and saber-toothed cats. In fact, I see dozens of species of large animals that no longer exist. But I also see a great number of familiar animals — mostly small creatures — that still live in North America. It is amazing to realize that so many species of “megafauna” lived in North America so recently, and that almost all of them have gone extinct — leaving behind a continent that is populated mostly with small animals.

After I reach the west coast, it dawns on me that I have not seen a single human being. Humans do exist in this time period, but not in the Americas. In fact, it was right around 100,000 years ago when modern humans (Homo sapiens) first began to migrate beyond Africa, spreading into Eurasia. Some very close relatives of humans, such as Homo erectus (now extinct), left Africa long before humans ever did — but they never reached North America.

I need to get a good night’s sleep, because tomorrow will be another big day for exploring.

Day 2 — 1 million years ago

This morning I have jumped back 10 times as far as I did yesterday, to a date that is a million years before modern times. A million years sounds like a very long time. And yet from a geologic standpoint, I still have not traveled very far back. I’m still in the Cenozoic era, and I’m still in the Pleistocene epoch — the same era and epoch that I visited in yesterday’s excursion. In other words, I’m still in the Ice Ages. Looking out the window of my flying time machine, the world still looks familiar. I still see forests and grasslands, with a mix of conifers, broad-leafed trees, grasses, ferns, and so on. If I squint I can imagine that I have not left modern times.

I park my craft to get a closer look. When I examine the plants up close, almost none of them are completely familiar. The same goes for the animal life. I can still see all the familiar categories of critters — mammals, birds, insects, reptiles, amphibians, and so on. I see a whole lot of plants and animals that look quite similar to modern plants and animals, but most of them are subtly different from their modern counterparts. Although a million years is long enough for quite a lot of evolution — not only new species, but some new genera too — life on Earth a million years ago was not hugely different from modern times.

I decide to fly across North America again, and I continue to see megafauna roughly similar to what I saw yesterday — but few of the species look exactly the same. Before the day ends, I make a quick hop over to Africa to check out the hominids there. I eventually find some humans — similar to modern humans but clearly smaller, and subtly different in several ways. In fact, I find several species of humans (Homo sp.) living in Africa and Eurasia. I wonder what it must have been like to share the world with several other closely related species.

Day 3 — 10 million years ago

For my third day, I have again gone back 10 times as far as yesterday — which is 100 times as far back as my first day. At 10 million years before modern times, I’m finally far enough back to escape the Pleistocene. I’m now in an epoch called the Miocene. However, I’m still in the Cenozoic era. I look around, and the world does indeed look different now — but not as different as I had expected. I still see forests and grasslands that look vaguely familiar. I don’t recognize any of the plant species, but I still see familiar types of plants, such as conifers, broadleaf trees, grasses, wildflowers, and ferns. Among the animals, all of the familiar categories are still here — mammals, birds, insects, reptiles, amphibians, and so on. What is eerie is that while I can still see all of the modern categories of plants and animals, nearly all of the individual species are unfamiliar to me.

After a while I finally realize why this world still seems somewhat familiar. It is dominated by the same types of plants and animals that dominate the modern world, even if not the same species. The vast majority of the larger animals I see are mammals — just as in today’s world. A very high percentage of the vegetation consists of angiosperms — flowering seed plants — just as in modern times.

I do a quick tour of the world, and it is fascinating to see early versions of so many familiar groups of creatures. I see a lot of mammals that are grazing on grasses — ancestors of modern ungulates (hooved animals). I see early canines. I see birds that are apparently early ducks, owls, crows, and so on — along with a few gigantic birds that are less familiar. I see several kinds of crocodiles, some of which are quite large. In the oceans I see an amazing variety of cetaceans — creatures related to modern whales.

I make another trip to Africa, but there are no humans — I’ve gone back way too far for that. So instead I find the apes that are the ancestors of humans. These same apes are also the ancestors of chimpanzees and gorillas. They don’t really look like any of the modern apes, and yet they bear a faint resemblance to all of them. I also encounter many other species of apes, some of which have no living descendants in the modern world.

Day 4 — 100 million years ago

Once again, I decide to go back 10 times as far as yesterday, this time into the Cretaceous period of the Mesozoic era. But now I think that I have jumped a bit too far, because the dramatic changes are quite overwhelming. I now see a world dominated by dinosaurs and other reptiles, living on a land covered by a very odd mix of plants. Most of these plants look rather “primitive”, although a few look surprisingly modern. Many of the unfamiliar plants are the size of trees, but there is also a lush understory of smaller plants. A wide variety of herbivorous dinosaurs are busy munching on the vegetation. I fly in closer to get a better look.

The most obvious creatures are the giant sauropods, weighing many tons each. With their long necks they can easily graze on the taller trees. I also see theropods — speedy animals that attack and eat other dinosaurs. Then there are ornithopods, many of which seem rather speedy in their own right. I also see ankylosaurs, slow but well-armed with giant spikes and extensive body armor. In the air I see quite a variety of flying pterosaurs — an impressive sight indeed. I also see a few flying species of theropods — the ancestors of birds. Two or three times I catch sight of some small furry creatures in the underbrush — early mammals. However, none of them are very big — certainly not like the mammalian megafauna I saw in the Pleistocene. Here in the Cretaceous, the megafauna is almost entirely reptilian.

I don’t dare step out of the craft for more than a few minutes, uncertain as to what creatures I might encounter. But when I do step out briefly, I find that the climate seems tropical. In the craft’s database, I see that the landmass that would later become North America is located fairly close to the equator. It was only later that North America drifted northward. I also learn that the entire planet is warm, and that there is no polar ice right now. Therefore sea level is quite high, flooding the lowland areas of all the continents with warm, shallow seas. Consequently there is a lot of sea life, including bony fish, sharks, ichthyosaurs, plesiosaurs, ammonites, squid-like belemnites, and nautiloids.

As the day comes to a close, I turn my attention to the plants again. Among the tree I see a lot of interesting conifers, many of which are similar to modern araucarias (such as the monkey-puzzle tree). I also see cycads and ginkgoes. I see many kinds of ferns, along with some horsetails, lycopods, and liverworts. All of these are ancient lineages of plants. But among the smaller plants, especially the shrubs, I also see a fairly broad variety of early angiosperms — the flowering plants that will later dominate the world.

Day 5 — 200 million years ago

Instead of going back 10 times as far as the day before, as I’ve done on my previous jumps, I’ve decided to go back only twice as far as I did yesterday. So here I am this morning at 200 million years ago, which puts me early in the Jurassic period of the Mesozoic era.

The world is again quite different — after all, I’ve jumped another 100 million years. And yet it is not as different as I would have thought. In particular, dinosaurs are still the dominant creatures — but a completely different set of dinosaurs than those I saw yesterday. These dinosaurs don’t seem to be nearly as diverse, and the largest are not nearly as large as some of the giants I saw yesterday. Many of the species I see are fast runners on two legs. I see a few kinds of pterosaurs flying in the air. And to my big surprise, I actually see a few tiny mammals. There are also some giant amphibians that I find quite fascinating. I make a brief stop at the edge of an ocean, where I spot some ichthyosaurs and plesiosaurs.

Among the plants, I see a lot of ferns, ginkgoes, and cycads. I see several kinds of conifers, although they are not nearly as dominant as they will become in the mid-Cretaceous. Angiosperms are almost non-existent, although I do find a couple of very early examples. It seems odd to think that I am visiting a world that is almost completely without flowers.

Day 6 — 400 million years ago

Just like yesterday, I decided to jump back twice as far in time as the previous day. So this puts me at 400 million years before modern times, in the Devonian period of the Paleozoic era. I am rather shocked at how different the world appears now. I hardly know how to describe it.

The land is covered with plants, but these are the strangest plants I have ever seen. The entire ground surface is covered with green, although some of the plants are as tall as small trees. And yet none of them seem to have leaves. I can best describe these plants as masses of branching stems. But they don’t all have the same branching pattern. Some consists of a large number of upward pointing forks. Others have a single straight trunk, with whorls of little branches sticking straight out to the sides. It seems that all of these plants reproduce by spores — I don’t see a single seed plant.

I see very few animals moving among these plants. There are certainly no dinosaurs or mammals. I don’t even see any amphibians among the plants. In fact, I don’t see a single vertebrate — an animal that has an internal skeleton. But I do see some arthropods that strike me as a bit creepy. Most of them look similar to millipedes, but I also see some that remind me of scorpions, and some that remind me of ticks or spiders. I quickly decide to get back in my craft to go check out the oceans.

I find that the oceans are full of interesting life. First of all, there is an amazing variety of fishes, many of which are quite different from modern fishes. So there are indeed vertebrates in this world, just not on the land. There are also brachiopods, which look like sea shells, and crinoids. The crinoids are a bit hard to describe, but they almost look like underwater flowers, even though they are actually animals.

Day 7 — 1 billion years ago

For my final day of vacation, I have jumped back in time to exactly one billion years ago, which is more than twice as far back as I was yesterday. This puts me in the Precambrian, in an eon called the Proterozoic. The contrast with yesterday is simply amazing. All of the land surfaces I see are stark naked. I don’t see any plants or animals anywhere, just bare soil. Actually, I see what looks like scum in any place that stays wet, so it’s not as if there isn’t any life at all. However, there is no living creature in the entire world that is large enough for me to see without a microscope. And yet life is abundant enough to accumulate into films of ooze and slime. I’ll have to admit — it doesn’t look very impressive to the naked eye. However, some of it looks quite interesting under a microscope — mostly unicellular creatures, but I do find a few that are multicellular.

What’s even more interesting is that I can see the underlying geology wherever I go — not covered in vegetation like the modern world. It’s a great opportunity to go rock hunting, and in fact that’s how I spend most of the afternoon, after I get tired of playing with the microscope.

As I look around the world in this time period, I feel as if I’ve gone back to the beginning of the Earth’s history — but that’s not really true. I’ve only gone back 1 billion years, to a world that was already 3.5 billion years old by that time. If I were to jump back another billion years, then the world would not look much different. However, the creatures in the slime would be simpler — only unicellular creatures, and hardly any with a nucleus.

Back Home

My vacation is now over, and I’ve returned the flying time machine to my good friend. She was relieved to see that I had not damaged anything. I think she was also relieved that I made it home safely, although she would not admit to having been worried about that.

My vacation has given me a lot to think about. I found it interesting that on my first day of travel, only 100,000 years ago, I could see a great number of giant mammals that no longer exist, even though there has been only minimal evolution in that time frame. It certainly demonstrated to me that extinction can occur on a much more rapid time scale than evolution does.

I also found it interesting that the world still looked quite familiar 1 million years ago, and reasonably familiar 10 million years ago. It wasn’t until I went back 100 million years that everything seemed quite different — a different world, so to speak, with dinosaurs and strange plants. And then at 400 million years ago, everything was completely different again — strange fishes in the seas, and even stranger plants on the land.

The biggest shock was seeing the bare world of a billion years ago, with no life beyond what looked like ooze and slime. It gave me a greater appreciation for the modern world, with its covering of green plants and its diversity of animal life. For my next vacation, I think I’ll stay in the present world.

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