A Journey Through Earth's Extinct Worlds
By Thomas Halliday
"It is the crucial paradox at the heart of paleontology that practically all our information about life comes only from death." This line from Thomas Halliday's Otherlands, of course, refers to the fossil record. So, it stands to reason that the entire field of paleontology is contingent upon microbes: whether microbes can or cannot access and completely decompose a dead organism. It's the latter path that allows a creature's record of existence to persist through time and to be happily discovered by paleontologists.
Drawing upon the fossil record and paleontological stratagems, Thomas Halliday conjures up lush and sweeping tableaus of ancient ecosystems in his book, Otherlands, beginning with the Pleistocene of 20,000 years past and working backwards through 550 million years to arrive in the watery Ediacaran. Each chapter is a window into one geologic era or epoch. For each time period, Halliday has picked a particular representative fossil site to breathe life into, and from such surviving evidence he deduces the movement of that period's flora and fauna, climate, weather patterns, cycling of water and soil and carbon. This reverse chronological portraiture of the shifting landscapes – lakes draining and refilling, continents pushing up against each other and buckling into mountain ranges, new plants and animals rising and fading – reminded me of a sculptor at work with clay in a stop-motion film, effortlessly graceful to the onlooker. All of Earth's contours move in fast-rewind in these pages.
This being Small Things Considered, I read with an eye out for the microbes. Through Otherlands' chronicling of Earth's past epochs (large things considered…), we can glean a bird's-eye view of microbial activity on Earth.
But first, patience – metazoans dominate the first half of the book. It opens with the Pleistocene epoch, in which permafrost gives way to bogs and meltwater rivers and taiga forests, roamed both by megafauna like mammoths and massive bears and smaller lifeforms like horses, birds, and insects (and the prehistoric humans responsible for the Lascaux cave paintings!). We are then wound back 4 million years to the Pliocene, witnessing the rise of C4 plants equipped with RuBisCO and of the early hominin Australopithecus. The Miocene from 5 million years ago sees the genesis of the Mediterranean basin, its creatures marked by dwarfism and gigantism; the Oligocene (32 mya) sees the preponderance of grasses; the Eocene (41 mya) the giant penguins and the sudden warming of the planet by carbon dioxide and methane; the Paleocene (66 mya) the resurgence of life after the asteroid impact at Chicxulub. Before that famed extinction event, there is the Cretaceous (125 mya), with theropods like the feathery Velociraptor of fame and the rise of the flowering angiosperms; the Jurassic (155 mya), with Earth's first birds and a massive reef of glass sea sponges three times the size of today’s Great Barrier Reef; the Triassic (225 mya), with landlubber dinosaurs and airborne pterosaurs and sea-dwelling plesiosaurs; and the Permian (253), in which life roamed atop Pangaea.
It's in the remainder of the book that microbes take the limelight. First stop here is the Carboniferous period (308 mya), with forests turning into peat turning into coal, a time when lignin in wood is novel enough that it is not yet easily broken down by microbes. In the Devonian (407 mya), fish evolve legs and venture onto land, where there are hot ponds inhabited by alkalithermophiles and cyanobacteria. Here, fungi enter into partnerships: with plants, to form the first mycorrhizae, and with cyanobacteria, to form the first lichens. In the Silurian (435 mya) oceans teeming with fish, microbes flourish around hydrothermal vents, also symbiotically associating with tube worms (as covered in previous STC posts, here and here), and the bacteria coating seafloor basalt fix large quantities of carbon as they still do today. In the Ordovician (444 mya), strange invertebrates swim through the seas and glaciers reshape the Earth, while algal blooms lead to constant marine snow that falls to the sea floor and nourishes the sulfur bacteria. In the Cambrian seas (520 mya), there are a lot of multicellular firsts: the first vertebrates, the first eyes, the first predators, the first hard body parts wrought from calcium carbonate, and the first known creature to brood and protect its eggs. Finally, the Ediacaran (a whopping 550 mya) sees a defrosting Snowball Earth whose seas are ruled by microbes and sessile creatures. Cyanobacteria thrive in these oceans – plentiful prey for newly multicellular algae – and the seabed is coated by microbial mats and stromatolites. And life even further back? "In the endless outback of early Earth history… are only microbes."
I loved this book. What is unfamiliar is made knowable, and what is already familiar about life on Earth is made excitingly novel through views from unusual angles. It gave me a renewed appreciation for microbes as Earth's engineers since the dawn of life. As I read this rewind of Earth's eras and epochs, I recalled previous STC posts, and I wondered: With all the insect diversity of the Cretaceous, how many of those insects were associated with Wolbachia, and did microbes play protective roles for their eggs as they do today? Did the springtails jumping about in the Devonian have such intertwined relationships with Streptomyces and geosmin? About those nematode worms that parasitized Devonian plants – what of microbial companions in their gut? Could fluctuations in populations of ice-nucleating bacteria have affected global cycles of freezing and thawing? And so on.
The writing itself is also beautiful. With the subject matter handled so thoughtfully not only at the level of meticulously detailed facts, as is already expected of science books, but even down to the construction of sentences, it's hard not to share the author's tender awe of Earth and its lifeforms. It is evocative writing. At times it's playful, even punny (a personal favorite phrase: "before molluscs took that mantle").
My only complaint: Halliday repeatedly attributes the origins of life on Earth to deep-sea alkaline vents. Of course, the competing argument for hot land-based ponds as the cradles of first life is compelling, and evidence in favor of this theory is gaining mass and traction. This alternate theory is not mentioned.
Regardless, Otherlands is a delightful tour-like read. What a tribute to Earth's life in all its past, present, and future forms. For those experiencing wanderlust but unable to travel, here is the solution.