Learning of life as never before

kw: book reviews, nonfiction, natural history, geological history, life, evolution

Hmm, let's see, will this do?

Probably not. The little blob represents an amoeba. When did amoebas first appear on Earth? Probably around 800 million years ago. Amoebas are quite advanced. Life began with pre-bacteria (or pre-archaea) cells some time between 3,500 and 4,200 million years ago, so eons of evolutionary progress occurred before protozoans such as amoebas came into being. On the scale of this diagram, there would be another 35-40 things strung out to the left, for about two feet, all looking a lot like the period at the end of this sentence, or maybe a tiny oval of similar size.

So, just how has life progressed since it first appeared on earth? To find out, read a wonderful new book by Dr. Peter Ward and Joe Kirschvink, A New History of Life: The Radical New Discoveries About the Origins and Evolution of Life on Earth. Geology has not stood still since I studied it in the 1970's and early 1980's. Neither has biology, nor genetics. In fact, our understanding of DNA and genetics has advanced more in the past decade than in all of prior history…and we realize that we still know something like 1% (or a lot less) of what we thought we would know by now.

Geological history and biological history go together. Only the first era of Earth, called the Hadean, during which there were no solid rocks, was without life. A period called Late Heavy Bombardment (LHB) ended the Hadean Eon and partially overlapped the first appearance of living cells. Life may have arisen a few times, only to be melted out of existence when a rocky body about the size of Texas would cause most of the crust to melt, again. So what we call "the origin of Earthly life" actually began just after the last successful total extinction event. The remaining history of Earth and of Earthly biology is divided into eras marked by partial extinction events. Five (plus one, shown below, in my accounting) are major, in that more than 50% of all living species were eliminated. Others caused many more extinctions in a short time than the background rate, but less than 50% at a blow.

To be clear: "Event" is a comparative term. When referring to geological time, anything that happens in less than a million years, if it is more than 50-100 million years in the past, can be called an event. So let us set a time scale, primarily of these extinction "events", based on what is currently known:

• 4,570 ma ("ma" means "millions of years ago") - Completion of Earth's accretion, and beginning of the Hadean Eon, during which the entire planet was molten, and which ended when solid crustal rocks began to form.
• 4,100 to 3,600 ma - Late Heavy Bombardment, when most of the craters on the Moon, Mercury and Mars were formed. One relic on Earth is possibly the Nastapoka Arc in Hudson's Bay in Canada. Anything smaller has been eroded away. The first glimmers of life, and earliest putative fossils, date from the end of this era. There are also chemical signals in rocks aged 3,800 ma, that indicate photosynthetic life existed at that time. The Archean Eon is considered to have begun 4,000 ma. The Last Total Extinction.
• 2,450 ma - Oxidation Catastrophe. During the Archean Eon life originated and soon became photosynthetic. For a billion years or more oxygen was immediately taken up by reduced minerals such green iron oxide (Ferrous Oxide, FeO2) and pyrite (FeS2), which produced red iron oxide (Ferric Oxide, Fe03) and iron sulfates such as FeSO4 and Fe2(SO4)3. Once all the reduced minerals had been oxidized, oxygen began to accumulate in the atmosphere, killing nearly everything. Those living things that evolved the ability to survive in the presence of oxygen, and later, to even use oxygen for producing cellular energy through respiration, took over the earth during the ensuing Proterozoic Eon. This was probably the nearest thing to a total extinction since the series of total extinctions at the end of the Hadean Eon. Great Extinction #1.
• 2,400-2,100 ma - Huronian Glaciation, probably the first "snowball earth" period, unless it was "only" a near-snowball, a "slushball" with a narrow equatorial unfrozen belt. A Significant Extinction Event.
• ~1,650 ma (maybe 2,000 ma) - Origin of Eukaryotes, large, complex cells and later on multicellular life composed of such cells; to be discussed later.
• 780 (720?)-635 ma - Three better-studied Snowball Earth extinctions of the Cryogenian Era, in the late Proterozoic Eon. Each would have made most of Earth unlivable for all but the hardiest creatures. Three Extinction Events, each lasting several million years.
• 542 ma - End-Ediacaran Extinction, A Significant Extinction Event. Early soft-bodied, multicellular creatures, and a few with hard parts, abruptly vanished. Possibly caused when predators arose that could eat the Ediacaran animals. The period that followed is the Cambrian Period, the first period of the Paleozoic Era of the Phanerozoic Eon, which is still going on.

The appearance of hard-shelled fossils just before the Cambrian Period makes it easier to distinguish changes in the fauna, particularly extinction events.

• 488 ma - late Cambrian Extinction Event (AKA SPICE, for Steptoean Positive Carbon Isotope Excursion, a technical designation indicating a dramatic chemical change in the atmosphere and ocean).
• 450-440 ma - Ordovician-Silurian events, a series of global cooling events, possibly ice ages, that wiped out 70% of species. Great Extinction #2.
• 375-360 ma - Late Devonian volcanism. Over a few million years, half of all animal and plant genera became extinct, and perhaps 75% or more of all species. Great Extinction #3.
• 252 ma - End Permian Mass Extinction, the greatest of the "Big 5", which actually starts with #2 in this list. A confluence of several causes, including inconceivably enormous amounts of lava that erupted to form the Siberian Traps ("trap" is a kind of volcanic rock). At least 90% of all species wiped out. Great Extinction #4.
• 201 ma - End Triassic Mass Extinction, triggered when a less intense extinction caused by trap volcanism was augmented by an asteroid impact. At least 50% of species became extinct. Great Extinction #5.
• 65 ma - End Cretaceous Mass Extinction, primarily caused by an asteroid impact, but trap volcanism was also occurring around this time in India (Deccan Traps). Wiped out most of the dinosaurs, leaving only birds, and also the pterosaurs and marine reptiles such as the plesiosaurs. The most famous of the "Big 4", Great Extinction #6.
• 2.5-0 ma - The ongoing Pleistocene-Holocene Mass Extinction. Initially a result of the ice ages that began when North and South America became connected at the isthmus of Panama, it is not yet clear whether this is, or will, rank with the "Big 5". What was begun by continental glaciation is apparently continuing due to human interference with the biosphere, possibly including causing a big boost in carbon dioxide. A Significant Extinction Event. If we outdo ourselves, it could become #7.

This is the framework within which life on Earth has developed and evolved. According to this Wikipedia analysis, these are the most significant of at least 25 extinction events. While Earth nurtures life, and may be a nearly unique planet in doing so, it is still a dangerous place. Just ask the more than 99% of all species that are no longer living here. In spite of that, there are between 5 million and 15 million species of Eukaryotic life in existence, and anywhere between a few million and a billion kinds (the word "species" is harder to apply) of Prokaryotic life.

Now it is time for terminology about the kinds of living cells.

• Prokaryote is meant to convey "prior to the nucleus", where "kary" refers to the nucleus in cells that have them. There are two major domains:
• Archaea, which until recently were considered to be bacteria. But they have an odd mix of primitive and advanced features that bacteria don't have at all. For example, their ribosomes, which translate DNA codes into proteins, are complex and very similar to those of Eukaryotes. Bacteria have simpler ones.
• Bacteria include all other prokaryotes, including all known pathogenic prokaryotes. Some (including me) consider Bacteria a younger offshoot of Archaea, and others consider it the other way around.
• Eukaryote means "good nucleus". The cells are much larger, and contain mitochondria, which are prokaryote-sized energy-producing organelles that are considered to be descended from prokaryotes that were engulfed by a larger one but, rather than being digested, kept, "enslaved", and eventually became an internal part of all eukaryotic cells. Alternatively, the smaller prokaryote may have begun as an endoparasite to a larger species, that developed into an endosymbiont. Anyway, every cell in your body (except red blood cells) contains from hundreds to thousands of these little organelles, and you'd die in a matter of seconds if they were somehow killed off.

The authors draw on new discoveries in every area of geological and biological sciences imaginable. The various eons, eras, periods and so forth, delineated in the outline above, had an array of living creatures, both plant, animal, fungal, and prokaryotic, that differed significantly from those in any other. For example, before about 120 ma, were you to visit in your pocket time machine, you could not go about smelling the flowers because the few that had existed for the prior 30-40 million years didn't have nectar or perfume yet. There weren't any bees to attract. Jump back to the middle Cambrian, say, 520 ma, and there was nothing living on land except some bacterial crusts here and there. The main sea-bottom creatures were scuttling shrimp-like and isopod-like things including trilobites, also mollusks and clam-like brachiopods. Tentacled things such as ammonites weren't yet present, but there were numerous creatures we would probably not recognize, scuttling and swimming about.

The kinds of living things that existed, and the transitions from one kind of biosphere to another afforded by the various cataclysms, form the main subject of the book. I am overwhelmed by the sheer mass of information the authors packed into some 360 pages. Even many of the end notes,  those that weren't just strings of references, made fascinating reading.

To pick one significant learning from the book as a whole: several of the extinction events are classified as "greenhouse events". Certain periods were characterized by temperatures beyond tropical. Given that the Sun was several percent less bright half a billion years ago, and 40% dimmer around 3,000 ma, carbon dioxide alone could not produce global temperatures that would have exceeded 35°C (95°F) nearly everywhere, day and night. Methane eruptions, possibly from warming of methane clathrates in the shallower seabeds, were involved.

Certain geologic cycles, such as the formation or breakup of a supercontinent such as Pangaea or Rhodinia, cause large excursions in the level of carbon dioxide in the atmosphere. While that level was around 280 ppm a century or so ago, and is 400 ppm today, it was 1000 ppm or higher, sometimes much higher, during much of prehistory. A period of warming when carbon dioxide rises can trigger methane release. Methane doesn't last more than a few years in the atmosphere, because it can be oxidized. But sometimes, oxygen has been low at the same time as high carbon dioxide and methane, and then the methane lasts much longer and accumulates. The Triassic contained one such ultra-hot period.

And a fun fact: The present oxygen content of the atmosphere is 21%. At certain earlier times it was as high as 35%, which had several effects: the amount of nitrogen was the same as ever, so air pressure was ~15% higher; the extra oxygen provided extra energy to animals, and could in particular penetrate more deeply into the semi-passive respiratory systems of insects; and these two things led to some insects getting very large, such as a dragonfly with a meter-wide wingspan. The atmosphere was smokier, though, because of great forest fires. But the oxygen level was kept high by the rapid burial of organic debris that didn't get much chance to rot during that period. So...a dragonfly as big as a crow. Wow!

You don't have to specialize in geology or biology to enjoy this book. It is intended for us all, and the authors are very good at explaining what their jargon means.

(I won't go into detail about a few copy-editing errors I ran across. That's for a private letter. But I must comment that good proofreading and copy-editing are becoming less common. Authors out there, and editors, you can't just rely on a spell-checker. How else to account for the word "Cambria" where "cambium" was meant? The former is the historic name of Wales, and the latter is the living tissue in a tree.)