The magnificent artwork and sketches are by Alain Bénéteau
abbreviation used in this article:
- mya: million years ago (before the present)
I originally decided not to review this exasperating - and wonderful - book: too small a potential audience, too specialized, too technical, some serious pedagogical issues..
However, perhaps against better judgement, here 'tis. Ultimately I was moved to write because I love the book and hope that a few readers will decide to check it out at their local library and enjoy it as thoroughly as I did. (If your library doesn't have it, recommend it to the holdings department. Most libraries welcome suggestions. I suggest several books per year and have been refused only twice. This way you get to read a good book for free and assure that others will have the chance to read it too.)
Subject: Early tetrapod (four-legged animal) evolution from the Devonian to the Permian mass extinction, 416 - 250 million years ago (mya). This is our history, human history, in deep time since humans are mammals and mammals are modern representatives of tetrapoda, the four-legged tribe.
The image above is hynerpteton, a Late Devonian tetrapod, 360 mya. Hynerpeton was an aquatic carnivore living in coastal mangroves, capable of limited motility on land (like a modern walrus or seal). Its primary mode of propulsion was the large tadpole tail, visible in juvenile frogs and the embryos of terrestrial vertebrates.
Before the dinos was originally published in French. Chris Spence, an ex-patriate American living in Paris, deserves due credit for the quality of the translation. I had no impression whatsoever that the text had been composed in a language other than English. Good job, Chris! The only gaffe I found was the translation of the Canadian province, Novia Scotia as "New Scotland"! Quite an odd error for a professional translator..
It is rare that science books, especially highly technical ones, are considered "beautiful" (as one reviewer was moved to remark). The quality of the artwork, in fact, moved me to undertake this review. At times it is stunning. The illustrator, Alain Bénétau, prefers dramatic "cloud-wracked" skies, threatening storm. On the whole the images are far more life-like (hence interesting) than one would expect for an academic work.
Some of the images achieve a quasi-photographic quality to the point one can imagine one is looking at a photo of an exotic rain forest critter in National Geographic. The image of archaeothyris, 310 mya, the "oldest known synapsid" (the ancient genetic lineage of which mammals are the sole survivors) could fool most people,passing off as a photo of a somewhat scary looking modern reptile, 20 inches in length.
The visuals are the book's strongest point. It is literally jammed full of images of all sorts: paintings, sketches and photos, body silhouettes, several per page. Many are photos / sketches of fossils, including in situ photos before removal and preparation of the fossil for display. Fossil photos are generally accompanied with one or more sketches outlining anatomical features of evolutionary importance. I found these a pleasant way to deepen both my knowledge and appreciation of early vertebrate evolution.
Ichthyostega (top) and acanthostega: early tetrapods. Ichthyostega had a robust shoulder girdle and front limbs which allowed it to hump about on land by alternately scrunching up its tail to push forward and then using its powerful front limbs to push back (the hindlimbs were useful for swimming only: too short and badly angled to walk with. Fossilized races of body dragging as described above are found in places where ichthyostega was abundant. Acanthostega (bottom), also an early tetrapod, was not to any degree adapted for locomotion on land).
The biggest, most glaring flaw of Before the dinos is the absence of a technical glossary: absurd for such a technical and highly specialized text, and particularly for one with pretentions of a larger readership. The quality artwork does, in fact, suggests an attempt to appeal to a larger readership, an impression furthered by the blurb on the front and back covers of the original French edition. In addition, prof Steyer included photos of field working conditions and modern paleontological technology (scanners, radio-isotopes..) both of which would be irrelevant if the intended readership was purely an academic or professional one. He also waxes idylically about the pleasures of a career in paleontology on several occasions: also irrelevant for a professional readership. The lack of an adequate glossary is thus as puzzling as it is irritating. I was eventually reduced to inserting book marks in places where technical terms were defined so I could flip back to refresh my memory when I got bogged down in a particularly dense thicket of technical verbiage: ichthyan sarcopterygian versus tetrapodomorph sarcopterygian for example (!!!) C'mon prof Steyer, give the reader a break..
Ichthyostega seizing prey 360 mya in a Devonian coastal mangrove. Note the submerged tree trunk..
History of the tetrapods, the four legged tribe: This history displays the strong dependence of organisms upon their environment and, by inference, the impact of the physical environment on biological evolution. (see footnote 1). Way back in the Paleozoic (early life) earth, the moon was much closer than it is today. This increases the gravitational forces raising tides. In addition, the earth rotated a bit faster, further increasing tidal forces and raising higher tides. The result was that the intertidal zones, the land impacted by daily incursions of tides, were much larger than today's intertidal zones. Marine mangrove ecosystems - intertidal forests - were much, much more extensive and diversified than today.
modern mangrove, where sea and forest meet
Paleozoic mangroves provided a rich, diversified, "fractally broken" network of partially interacting "microclimates", very favorable to evolutionary experimentation and diversification. (note 2) In this perspective the early tetrapods can be seen as highly mutant fish, intermediate in form and function to both fish and land animals. Thus tetrapods, as a group, possessed both lungs and gills. But why lungs?
In the light of current knowledge, lungs did not evolve in some attempt to "conquer the land". It is likely that they were used for dealing with oxygen impoverished water. The large intertidal zones of the Paleozoic, circa 350 - 400 mya, would have been characterized by very high tidal velocities, compared to today's oceans. This is reflected in the large amount of fossilized plant "litter" found in Paleozoic mangrove sites. The litter would have provided nutrients for abundant bacterial blooms, particularly on warmer, subtropical and tropical seacoasts. The bacterial blooms would have de-oxygenated the water, necessitating the evolution of a backup system of respiration: swim bladders modified to serve as lungs. The development of air-breathing was not, therefore, an adaptation to life on land or a "stage" in the "conquest of the land". Lungs and air-breathing were merely an adaptation to an oxygen deprived aquatic environment. Then, since life is opportunistic, only later did some of the critters re-adapt their capacity to breathe air to spend more time on land. The "fractally broken" mangrove, comprised of alternating stretches of shallow swamp and land, could be better exploited by amphibious tetrapods capable of negotiating short stretches of terra firma separating adjacent swamps. They were still aquatic predators: living on the land "for real" would only come aeons later.
Chiridian limbs, you say? The greatest innovation of the tetrapods, when they were still aquatic (be it noted), was probably "chiridian limbs". These are the limbs of land animals (including cetaceans - whales and dolphins - who returned to the sea). The primary feature of interest here is the termination of chiridian limbs in bony digits as opposed to the cartilaginous rays typical of fishes. See graphic below. Typically, the early aquatic tetrapods and tetrapodomorphs ("close-to-tetrapods"), had 6, 7, or 8 digits per limb, probably reflecting their usage in swimming (big webbed foot to displace a lot of water at each stroke).
Ichthyostega and the beginning of chiridian limbs, the word "rayons" in the leftmost limb sketch is French for the cartilaginous rays of fish fins. Panderichthys shows a curious hybrid form found in some early tetrapodomorphs: the bones of the limb are analogous those of land animals (modern tetrapods) while the cartilaginous rays are typical of modern fish!! A "missing link" (sort of).. Click on the image for a larger more readable version.
In reality, no one knows for sure why chiridian limbs evolved in the first place: no one can identify the "selective pressure" driving their evolution. The very earliest chiridian limbs appear on critters whose anatomy did not permit them to crawl about on land. (When critters began spending some time on land, then chiridian limbs, with their robust terminations did indeed prove useful - but they did not evolve for that purpose.) Prof Steyer proposes that they might have been used for seizing and holding the female during mating but admits this is pure speculation.
Another suggestion I have come across: chiridian limbs allowed tetrapod predators to sneak up on their prey, walking along the cluttered bottoms of mangrove swamps, concealed by bacterial bloom clouded water. Their prey, like other fish, would have possessed "lateral lines" with sensory endings designed to detected the vibrations set up by the fins of approaching predators. Sneak-walking along the bottom would have produced less low frequency vibrations than swimming. Another possibility (from myself): some of the early tetrapodomorphs and tetrapods have flattened bodies and eyes on the backs of their heads. This suggests that they concealed themselves on the bottom waiting for prey to come by. If so, they could have used their chiridian limbs as shovels and fans to spread mud and fine sand over themselves for concealment..
Missing links - real or imaginary? Prof Steyer rejects the notion of evolutionary "missing links" because, he believes, evolution has no goal, no final purpose. There is no "plan" directing evolution "from" species A to B to C with B the "missing link" between an earlier, more "primitive" form, A, and the "more evolved" form, C. In reality, Steyer argues, when we speak of missing links we are confounding familial likeness with an ordered (intentional) sequence of steps. Similarity in the fossil record merely reflects genetic descendance, not the unfolding of an "evolutionary project".
From Steyer's "democratic" interpretation of evolution, each species is its own "goal" in the sense that its members seek to survive an propagate their kind. In this perspective, humans are not really "superior" to other life forms. Rather, each form of life possesses its own specificities, its own "essence" or mode of "being-in-the world". Its specificity allows a species to occupy / create an ecological niche which it will occupy until changing environmental / ecological conditions render it "obsolete" and it will be replaced. Ichthyostega was not a "stage" on the road to becoming-human. It was not even out to "conquer the land". Its chiridian limbs provided some mobility on terra firma, allowing this predator to extend its hunting range. It could cross a stretch of dry land to get to the next wet piece of mangrove swamp. With improved mobility, it prospered until environmental conditions changed or until a competitor better adapted to its environment appeared. The only reason ichthyostega bears features in common with tetrapods, mammals and humans is because it belonged to a successful group of species that left descendants, including ourselves. Ichthyostega was merely doing its thing: predating, fleeing, fighting mating, not "striving" to give birth to humanity, a third of a billion years later..
Comment: In principle, prof Steyer is - most likely - correct in denying an overall purpose or identifiable goal in biological evolution (at least as a first approximation to reality). Nevertheless, this should not blind us to the possibility - the likelihood! - that certain "evolutionary strategies" prove more successful than others. Thus vertebrate evolution has established several lineages (birds and mammals) in which "cephalization" has proven a successful "strategy" over time. The brain becomes larger and more complex despite the enormous drain on metabolic resources (oxygen, energy) required to maintain a large brain. The increased behavioral adaptability a large brain permits can, in some environments, offset the increased metabolic demands of feeding a big brain. Minaturization is possibly another emerging evolutionary trait in vertebrates. As the nervous system evolves, cooperation between increasingly intelligent individuals becomes increasingly advantageous from a survival standpoint: two heads are better than one (especially if both are smart..). In this scenario, cooperating individuals have a lowered mortality, hence greater ability to pass their smart genes on to offspring. Each generation will, infinitesimally, tend to be brighter than the previous. Now, this situation generates "selective pressure" for greater population densities: a denser population permits more cooperative interactions between individuals. Unfortunately, natural ecosystems can only feed so many individual per square hectare. A way around this bottle neck - a new "selective pressure"! - is to reduce the average size of individuals thus allowing a greater population density for the same amount of food consumed. If the competitiveness of the species is improved, selection will lead, over time, to smarter, smaller individuals. Some evolutionists believe there is evidence for such miniaturization of modern, highly cephalized vertebrates.
How modern tetrapods, upper right, are related to fish, left. All the critters between lung fish and modern tetrapods - "the missing links" - have gone extinct.. Again, click on the image for a more readable enlargement.
Moradisaurus grandis, late Permian herbivorous reptile
Note the large defensive talons - "don't mess with me!"
1- Climate change as a - or the - major "driver" of bilogical evolution on earth:
internal blog links: keywords: science, book review, climate change
2- The following quote is from a recent book review of a text on Disturbance Ecology which deals with the biological diversity fostered by fragmented, frequently disturbed ecosystems:
"Disturbances help generate the mosaic makeup of the habitat. A fire burns out a patch of forest and open it up to sunlight. Now, small plants, which had been suppressed by the shade of the trees, can thrive, and then a meadow can develop. Every organism is uniquely adapted to a particular type of habitat and a diverse array of habitats can support many more species than a uniform habitat. A variety of habitat patches, in turn, supports a diversity of species and communities. This biodiversity is the foundation of the natural ecosystem services upon which all life depends. Contrary to common thinking, disturbances are not bad, but rather they are valuable - indeed, they are essential for healthy ecosystems.. The nature of nature is change."