A S H O R T H I S T O R Y O F V I S I B L E L I F E O N L A N D ---Preface--- The second book I thought of writing was a politically correct history of nature. In other words, I wanted to present the origins of, say, the mosquitoes, which could hardly have been in the "age of insects" (nothing then had blood for them to suck!), and not just of mammals. I've found that the fossil record won't support a truly politically correct history. Insects are, anyway, relatively well documented, but spiders are not, and spiders are in this regard much more typical. So I decided to write this up now instead. ---Introduction: Two Classifications--- There are two sets of technical terms that I need to use in this account. One is geological, or at least palaeontological; the other is biological. The geological one is how the "geological time scale" is structured. Basically, geologists divide the years up using five kinds of division: eons, eras, periods, epochs, and ages. The normal way of dividing these is actually palaeontological in nature, because most rocks are recent enough to have fossils in them, and it turns out that fossils are more often usable to date a particular rock than half-lives of isotopes, or similar things. So basically, when the fossils change a lot, you get a new age, epoch, or period; when they change wholesale, you get a new era or eon. In practice, there are two reasonable-length eons (the first and the one we're in now) and two others that are just arbitrarily divided in the middle; eons run to the high hundreds of millions of years, or more. The current eon and the two preceding it also have eras. Eon #3 has really long eras, but eon #2 and eon #4 agree that an era should last something like the low hundreds of millions of years. The current eon and eon #3 also have periods. Eon #3 again goes large; periods in our eon last roughly the high tens of millions of years. Finally, our eon has epochs (the low tens of millions of years) and ages (typically a few million years, though some are longer or shorter). We are in the Phanerozoic eon, eon #4 since the Earth began, and we are 542,000,000 years into it. We are in the Cenozoic era, era #3 since the Phanerozoic began, and we are 65,500,000 years into that. We are in the Neogene period (the term Quaternary is now unofficial), period #2 since the Cenozoic began, and we are 23,030,000 years into that. We are in the Holocene epoch, epoch #4 since the Neogene began, and we are supposedly 11,500 years into that (though in fact it's more like 13,500). The Holocene is too new to be divided into ages yet. To simplify things in the following, I'm going to use a uniform way of representing years, in millions measured back from today. So 542 Mya means "542,000,000 years ago". (In chronologies, 542 means the same thing.) This document won't cover the most recent million years, so I shouldn't need to get any more precise. Geology has had a revolution in recent decades - the total acceptance of the theory of continental drift, which made plate tectonics central to the discipline - but it hasn't had a huge impact on the geological timescale's setup. This contrasts sharply with the impact the genetic and biochemical revolution in biology has had on the setup used by taxonomy. Taxonomists use *lots* of ways to categorise groups of creatures, but at bottom there are seven: kingdoms, phyla (divisions in plants), classes, orders, families, genera, species (not all kinds of creatures have meaningful species). What these mean is, approximately, thus: A species can all interbreed, like people. The species in a genus are more or less the same: chimpanzees and pigmy chimpanzees, say. The genera in a family are recognisably related, more or less the same kind of critter: cats, tigers, and lynxes, say. The families in an order are anyway sort of the same *kind* of thing: spiders, for example, are an order. The orders in a class should be roughly similar at the level of deep structure: birds, for example, are a class. The classes in a phylum should have the fundamentals in common: flowering plants, for example, are a phylum (or anyway a division). And the phyla in a kingdom should, well, that's where the trouble is. Basically every level from classes up has been in serious dispute in recent years both because of changes in how people think about classifications, and because genetic evidence sometimes conflicts with the sort of anatomical evidence that was previously traditional. One of the most spectacular examples of this conflict is that some would argue that there are three kingdoms (and not the three you're thinking of, either), and others would argue that there are several dozen. That said, most of our own classification is relatively uncontroversial. We are species Homo sapiens, human beings. We are in genus Homo, roughly, people. We are in family Hominidae, roughly, people and extinct people-like critters. We are in order Primates, which ranges from lemurs to monkeys to apes to us. We are in class Mammalia, warm- blooded creatures that suckle their young. We are in phylum Chordata, all of which have spinal cords at least part of the life cycle. And we are in kingdom Animalia, generally agreed to be a Real Kingdom (tm). OK, all done now. ---Prologue: Life and Land Before They Met Each Other--- There has only been life on land for less than 500,000,000 years, that is, less than one ninth of the time the Earth has been around, and less than one seventh of the time life and land have been around. I don't intend in this document to deal with all kinds of life, so I don't want to go into tons of detail here, but I do think it's worth conveying some basic facts. There are plenty of books out there that attempt this, but as it turns out, there aren't too many facts about the other eight ninths of Earth's history that will excite the average reader, so these books tend to bury those facts in stories about how scientists work or speculations that aren't well grounded. In order to keep myself from doing the same, I'm going to present this chronologically. I don't know of anyone who's done a serious chronology of what's known about the "early" (well, non- late) Earth anyway. Note, as you read, that I use plenty of words to indicate differences between relatively well-grounded facts, and speculations. If Fact X "means" Y, Y is a pretty solid bet, but not a guarantee; if Fact X "hints at" Y, Y is a speculation that I happen to like. At least in the early stages of the chronology, most of the time, Fact X "suggests" Y, or something similar, or simply Y "is supposed" to be true; this is somewhere in between these two examples. THE HADEAN EON This is the one unofficial unit of time I'm using here. Although it is not recognised by the International Union of Geological Scientists, the Hadean just makes too much sense (especially as compared with the following two eons) to not use, and many others do use the term. Those stuffy enough to avoid it would instead call it part of the Eoarchean era of the Archean eon. The Hadean (not the Eoarchean) is defined as beginning when the Earth did - which appears to be sometime close to 4567.1 Mya - and ending (according to one sensible view) when life arose. It is only in recent years - make that, the past five - that rocks have been found that originated on Earth in what may be the Hadean. So we know very little about the period. We know enough, however, to imply that it very often deserved its name (after the Greek god of Hell). Chronology: 4567: Oldest known carbon-aluminum inclusions in meteorites. This is taken as a good estimate for when the solar system originated, in the sense that carbon and aluminum are important components of (in particular) Earth. (Serious astronomical books about the Sun base their statements about *its* age on these same meteorites, too, although carbon and aluminum are, well, not important components of the Sun. Caveat emptor.) 4555: Oldest known moon rocks. The Moon is generally supposed to have resulted from a planet about the size of Mars colliding with the nascent Earth, but I don't know if these rocks could date from before the collision. 4467: Current models say the Earth should have reached roughly its present size - should have finished accreting - within a hundred million years; in other words, by this time. Presumably the proto- Earth/proto-Moon collision was well before this. 4363: Probable date for some zircons from western Australia. Zircons should dissolve in most magma, so this suggests that at all times since then it has been possible for rocks not to be in contact with magma. 4055: There is a rock formation in Canada, near the Slave Lakes (hence called the Slave Craton), whose oldest dated rocks are this old. They're igneous, so they tell us nothing about life, but they do show that it has been possible, ever since, for continent-type rocks to exist. (Continents are *much* more stable than modern ocean floor, none of which is over 200 million years old, so this means there was, by this time, something other than modern-ocean- like ocean in the world. It probably wasn't yet even a craton, which means basically a large piece of a continent, let alone a full-blown continent, but presumably the geological processes that produced the early continents were already operating. While I'm lecturing about continents, I should note that continents generally don't stick entirely out of the sea; in fact, we presently have *much* more land area in the world than has been normal even in recent times, because we're in an ice age. The definition of "continent" has to do with geology, not with where the sea level happens to be.) In imperfect searching, I've found no other reports of rocks that are older than the oldest evidence of life, and so potentially Hadean. 3900: The standard wisdom is that there are about forty craters on the Moon that are roughly this old, and this is usually understood to mean that either there was still a lot of stuff banging into planets at this time, or else there was a specific catastrophe at this time. Either way, if these craters really are all contemporary (which is disputed), and if Earth took a similar beating (presumably likely), then the ocean would probably have boiled at least once, and any hypothetical life would presumably have died. 3760: There are metamorphosed sedimentary rocks in western Greenland at least this old. They have bits of graphite in them whose carbon isotope ratios suggest that they are biological remains, and indeed that some sort of biological photosynthesis was going on. (Not all kinds of photosynthesis produce oxygen, however, and while whatever photosynthesis went into these rocks *could* have been producing it, we do know it wasn't getting into the atmosphere much. Whatever the atmosphere had in it at this time, which is much debated, it wasn't free oxygen.) These rocks cannot be Hadean if they do contain evidence of life, so the Hadean was presumably over by this time. Separately, the fact that these rocks are sedimentary strongly implies that there were rocks by this time that stuck out of the water, that there was water for them to stick out of and to wear them down, and so forth. Basically, proto-cratons taller than the ocean should have existed, and so should the ocean, and so should rain.