Energy and the Human Journey: Where We Have Been; Where We Can Go - Wade Frazier

[Magda Hassan]

Complex Life Colonizes Land

World map in late Devonian (c. 370 mya) (Source: Wikimedia Commons) (map with names is here)

Chapter summary:

• Ordovician expansion
  
  
• Second Tethyan ocean begins forming
  
  
• First modern-looking reefs form
  
  
• Rise of mollusks
  
  
• Gigantism and dwarfism
  
  
• Dominance of nautiloids
  
  
• First land plants appear
  
  
• Ordovician extinction
  
  
• Ice age begins and ends
  
  
• Reefs greatly expand in Silurian
  
  
• First land animals appear
  
  
• Fish develop first jaws
  
  
• Silurian extinction events
  
  
• Bony fish appear in Devonian
  
  
• Fish migrate to land
  
  
• Amphibians appear
  
  
• Lignin allows for vascular plants and trees to develop
  
  
• Seed plants appear
  
  
• Trees appear
  
  
• Challenges of migrating to land
  
  
• Plants create riverbanks
  
  
• Oxygen levels crash, making breathing difficult for new migrants to land
  
  
• Land ecosystems pull carbon from atmosphere, which initiates ice age
  
  
• Devonian reefs become greatest in Earth's history
  
  
• Series of Devonian extinction events
  
  
• End-Devonian extinction halts vertebrate migration to land
  

With the extinction that ended the Cambrian Period, animal life's greatest period of innovation was finished, but the next geological period, the Ordovician (c. 485 to 443 mya), still had dramatic changes. The Ordovician would not see any new phyla of note, but the Ordovician was a time of great diversification, as new niches were created and inhabited. They reached modern levels of abundance and diversity. Food chains became complex and could be called food webs. More so than the Cambrian Explosion, the Ordovician "explosion" was an adaptive radiation.[218]
The continental configuration when the Ordovician began was like the Cambrian's, with shallow hot tropical seas. The Paleo-Tethys Ocean began forming in the Ordovician. The first reefs that would impress modern observers were formed in the Ordovician. Different animals built the corals (1, 2, 3) than Cambrian reef builders; but there were no schools of fish swimming around them, as the Ordovician predated the rise of fish. Fish existed (1, 2, 3), but they were armored, without jaws, and lived on the seafloor. The first sharks may have appeared in the Ordovician, but because they had cartilaginous skeletons, the fossil record is equivocal. Some fish had scales, and an eel-like fish might have even had the first teeth. Teeth and claws were early energy technologies; energy applied by muscles could be concentrated to hard points or plates that could crush or penetrate other organisms or manipulate the environment.
Planktonic animals became prevalent, and were critical aspects of the growing food chains. Trilobites and brachiopods flourished, but the Ordovician's most spectacular development might have been the rise of the mollusk. Bivalves exploded in number and variety, and nautiloid cephalopods became the apex predators of Ordovician seas, and some were gigantic. One species reached more than three meters long, and another reached six meters or more. The largest trilobite yet found lived in the late Ordovician. Below is an artist's conception of the Ordovician seafloor. (Source: Wikimedia Commons)

Gigantism is a controversial subject. Islands often produce giant and dwarf species, and result from energy dynamics; in general, on islands, large species tend to get smaller and small species tend to get larger. A landmark study of polar gigantism among modern seafloor crustaceans concluded that the oxygen level was the key variable.[219] Recall that colder water can absorb more oxygen. Size is a key "weapon" used in evolution's arms race. The bigger the prey, the better it could survive predation, and the bigger the predator, the more likely it would kill a meal. Since the 1930s, there have been continual controversies over size and metabolism, energy efficiency, complexity, structural issues such as skeleton size and strength, and so on.[220] In its final cost/benefit analysis, complex life decided that bigger was better, and the Ordovician saw much larger animals than the Cambrian's. Bigger meant more complex, and more complexity meant more parts, usually more moving parts, and those required energy to run. Whether increasing size was due to more oxygen availability, more food availability, greater metabolic efficiency, reduced risk of predation, or increased predatory success, it was always a cost/benefit analyses and the primary parameter was energy; how to get it, how to preserve it, and how to use it.[221] The "analysis" was probably never a conscious one, but result of the "analysis" was what survived and what did not.
Peter Ward suggested that the superior breathing system of nautiloids led to their dominance.[222] Nautiloids do not appear in the fossil record until the Cambrian's end. Only one family of nautiloids survived the end-Cambrian extinction, and they quickly diversified in the Ordovician to become dominant predators and they replaced arthropods atop the food chain. During the Ordovician, nautiloids developed a sturdy build and they began spending time in deep waters, where their superior respiration system enabled them to inhabit environments that would-be competitors could not exploit.
Although the Ordovician's shallow seas were fascinating abodes of biological innovation, of perhaps more interest to humans was the first colonization of our future home: land. Land plants probably evolved from green algae, and although molecular clock studies suggest that plants first appeared on land more than 600 mya, the first fossil evidence of land plants appeared about 470 mya, in the mid-Ordovician, which would have been moss-like plants, and they seem to have preceded land animals by about 40 million years.[223]
The Ordovician was characterized by diversification into new niches, even creating them, but those halcyonic times came to a harsh end in one of the Big Five mass extinctions: the Ordovician-Silurian mass extinction. The event transpired about 443 mya, and was really two extinction events that combined to comprise the second greatest extinction event ever for marine animals. About 85% of all species, nearly 60% of all genera, and around 25% of all families went extinct.[224] The ultimate cause probably was the drifting of Gondwana over the South Pole, which triggered a short, severe ice age. As our current ice age demonstrates, ice sheets can advance and retreat in cycles, and they appeared to do so during the Ordovician-Silurian mass extinction. There is evidence that the ice age was triggered by the volcanic event that created the Appalachian Mountains. Newly exposed rock from volcanic mountain-building is a carbon sink due to basalt weathering (as contrasted with silicate weathering volcanoes spew basalt) of that fresh volcanic rock. The combination of Appalachian volcanism ending and subsequent sequestering of atmospheric carbon dioxide may have triggered an ice age. The ice age waxed and waned for 30 million years, but the first event was calamitous.
Two primary events drove the first phase of the Ordovician-Silurian mass extinction: the ice age caused the sea level to drop drastically and the oceans became colder. When sea levels fell at least 50 meters, the cooling shallow seas receded from continental shelves and eliminated entire biomes.[225] Many millions of years of "easy living" in warm, shallow seas were abruptly halted. Several groups were ravaged, beginning with the plankton that formed the food chain's base. About 50% of brachiopod and trilobite genera went extinct in the first phase, and cool-water species filled the newly vacant niches. Bivalves were largely found in seashore communities, were scourged when the seas retreated, and lost more than half of their genera. Nautiloids were also hit hard, and about 70% of reef and coral genera went extinct. The retreating seas somehow triggered the extinctions, and whether it was due to simply being exposed to the air or changing and cooling currents, nutrient dispersal patterns, ocean chemistry, and other dynamics is still debated, and those extinction events are being subjected to intensive research in the early 21[SUP]st[/SUP] century.
After as little as a half-million years of bedraggled survivors adapting to ice age seas, the ice sheets retreated and the oceans rose. The thermohaline circulation of the time may have also changed, and upwelling, anoxia, and other dramatic chemistry and nutrient changes happened. Those dynamics are suspected to be responsible for the second wave of extinctions. There also seem to have been hydrogen sulfide events.[226] Atmospheric oxygen levels may have fallen from around 20% to 15% during the Ordovician, which would have contributed to the mass death. Seafloor anoxia seems to have been particularly lethal to continental-shelf biomes, possibly all the way to shore. It took the ecosystems millions of years to recover from the Ordovician-Silurian mass extinction, but basic ecosystem functioning was not significantly altered in the aftermath, which is why a mass extinction during the Carboniferous has been proposed as a more significant extinction event. The first major oil deposits of the Middle East were laid down by the anoxic events that ended the Ordovician. Most oil deposits were formed in the era of dinosaurs, and the processes of oil deposit formation were similar; they were related to oceanic currents, and when they came to shore via the bottom, and the prevailing winds blew the top waters offshore, it became a nutrient trap, and anoxic sediments could form. When the winds blew onshore and left via the bottom, the waters became clear, and are known as nutrient deserts. The oscillation between nutrient traps and nutrient deserts can be seen in oil deposit sediments.[227]
The Silurian Period, which began 443 mya, is short for the geologic time scale, lasting "only" 24 million years and ending about 419 mya. The Silurian was another hot period with shallow tropical seas, and Gondwana still covered the South Pole. But the ice caps eventually shrank, and that shrinkage played havoc with the sea level and caused minor extinction events (1, 2, 3), the last of which ended the Silurian and also created more Middle East oil deposits. Reefs made a big comeback, extending as far as 50 degrees north latitude (farther north than where I live in Seattle). According to the GEOCARBSULF model, oxygen levels rose greatly during the Silurian and rebounded from a low in the mid-Ordovician; it may have reached 25% by the early Devonian, which followed the Silurian. Coincident with rising oxygen levels, more giants appeared. Scorpion-like eurypterids were the largest arthropods ever, and the largest specimen reached nearly three meters near the Devonian's oxygen highpoint. The first land-dwelling animals - spiders, centipedes, and scorpions - came ashore during the Silurian between 430 mya and 420 mya, and the first insects appeared about that time, and all of the first insects flew.[228] Beetles first appeared in the fossil record in the late Carboniferous. Arthropods became dominant predators once again, although cephalopods patrolled the reefs as apex predators. Brachiopods reached their greatest size ever at that time, although the succeeding Devonian Period has been called the Golden Age of Brachiopods.[229] As oxygen levels rose, trilobites lost segments and, hence, gill surface area, which may have been an ultimately extinctive gamble. When the Devonian extinction happened during anoxic events, trilobites steeply declined and thereafter only eked out an existence until the Permian extinction finally eliminated them from the fossil record. Fish began developing jaws in the Silurian, which was a great evolutionary leap and arguably the most important innovation in vertebrate history. Jaws, tentacles, claws… prehensile features were advantageous, as animals could more effectively manipulate their environments and acquire energy. On land the colonization began, as mossy "forests" abounded, and the first vascular plants made their appearance, although they were generally less than a hand-width tall when the Silurian ended, and nothing reached even waist-high.
Oxygen levels appeared to keep rising into the early Devonian (c. 419 mya to 359 mya) and then declined over most of the period. The Devonian marked the dramatic rise of land plants and fish, in what is called the Golden Age of Fishes, and that period saw the first vertebrates that enjoyed a terrestrial existence. Armored fish supplanted arthropods and cephalopods during the Devonian as the new apex predators and weighed up to several tons. Sharks also began their rise. The Devonian has been called the Golden Age of Armored Fish.[230] Rising oxygen levels have been proposed as causing the spread of plants and large predatory fish, and a school of thought challenges high-oxygen reasons for many evolutionary events. Nick Butterfield is a prominent challenger.[231]
Bony fish (both ray-finned and lobe-finned) first appeared in the late Silurian and thrived in the Devonian. All bony fish could breathe air in the Devonian, which provided more oxygenated blood to their hearts.[232] Ray-finned fish largely lost that ability and their lungs became swim bladders, which aided buoyancy, like gas-filled nautiloid shells. Ray-finned fish can respire while stationary (unlike cartilaginous fish, and sharks most famously) and are the high-performance swimmers of aquatic environments; they comprise about 99% of all fish species today, although they were not dominant during the Devonian. All fish devote a significant portion of their metabolism to maintaining their water concentrations. In salt water, fish have to push out salt, and in fresh water, they have to pull in water, using, on average, about 5% of their resting metabolism to do so. Brine shrimp use about a third of their metabolic energy to manage their water concentration.
Today's lungfish are living fossils that first appeared at the Devonian's beginning, which demonstrates that the ability to breathe air never went completely out of fashion. That was fortuitous, as one class of lobe-finned fish developed limbs and became our ancestor about 395 mya. The first amphibians appeared about 365 mya. In the late Devonian, lobe-finned and armored fish were in their heyday. The first internally fertilized fish appeared in the Devonian, for the first mother that gave birth.[233] A lightweight descendent of nautiloids appeared in the Devonian, and ammonoids subsequently enjoyed more than three hundred million years of existence. They often played a prominent role, until they were finally rendered extinct in the Cretaceous extinction. Nautiloids retreated to deep-water ecosystem margins and still exploit that niche today.
Land colonization was perhaps the Devonian's most interesting event. The adaptations invented by aquatic life to survive in terrestrial environments were many and varied. Most importantly, the organism would no longer be surrounded by water and had to manage desiccation. Nutrient acquisition and reproductive practices would have to change, and the protection that water provided from ultraviolet light was gone; plants and animals devised methods to protect themselves from the Sun's radiation. Also, moving on land and in the air became major bioengineering projects for animals. Breathing air instead of water presented challenges. The pioneers who left water led both aquatic and terrestrial existences. Amphibians had both lungs and gills, and arthropods, whose exoskeletons readily solved the desiccation and structural support problems, evolved book lungs to replace their gills, which were probably book gills.
All such developments had to happen in water, first, for a successful move to land.[234] The evidence seems to support the idea that life first began to colonize land via freshwater ecosystems, which provided a friendlier environment than seashores do. The first arthropods ashore were largely detritivores, eating dead plant matter, and what followed added live plants and early detritivores to their diets.[235] The land-based ecosystems that plants and arthropods created became nutrient sources that benefited shoreline and surface communities, but the vertebrate move to land was not initiated by the winners of aquatic life. To successful aquatic animals, the shore was not a new opportunity to exploit but a hazardous boundary of existence best avoided. Tetrapodomorphs probably made the vertebrate transition to land as marginal animals eking out a frontier existence.[236] The fins that became limbs originally developed for better swimming, and further muscular-skeletal changes enabled them to exploit opportunities on land. Two key reasons for the migration onto land may have been for basking (absorbing energy) and enhanced survival of young from predation (preserving energy).[237] The five digits common to limbed vertebrates were set in this time; early tetrapodomorphs had six, seven, and eight digits, and the digital losses were probably related to using feet on land.[238]
But plants had to migrate before animals did, as they formed the terrestrial food chain's base. Along with desiccation issues, plants needed structures to raise them above the ground, roots, a circulatory system, and new means of reproduction. Large temperature swings between day and night also accompanied life on land. Plants developed cuticles to conserve moisture, a circulatory system that piped water from the roots up into the plant and transported nutrients where they were needed, and plant photosynthesis needed water to function. Vascular plants pumped water through their tissues in tubes by evaporating water from their surface tissues and pulling up more new water behind the evaporating water via the "chain" of water's hydrogen bonds. The last common ancestor of plants and animals reproduced sexually, and sexual reproduction is how nearly all eukaryotes reproduce today, although many ways exist to reproduce asexually. The first vascular plants are considered to have attained their height in order to spread their spores.[239] The Rhynie chert in Scotland is the most famous fossil bed that records complex life's early colonization of land.
The early Devonian was a time of ground-hugging mosses and a strange, lichen-like plant that towered up to eight meters tall. The oldest vascular plant division ("division" in plants is equivalent to "phylum" in animals) still existing first appeared about 410 mya, and today's representatives are mostly mosses. In the late Devonian, horsetails and ferns appeared and still exist. Seed plants also developed in the late Devonian, which enabled plants to quickly spread to higher and dryer elevations and cover the landmasses, as seed plants did not need a water medium to reproduce as spore-based systems did. In spore systems, which are partly asexual but have a sexual stage, a water film was required for the sperm to swim to the ovum. The first trees appeared about 385 mya (1, 2), could be ten meters tall, and formed vast forests, but reproduced with spores and so needed moist environments. The first rainforests appeared in the Devonian and reached their apogee in the Carboniferous. Those rainforests produced Earth's first thick coal beds. The Devonian was the Cambrian Explosion for plants, and enabled animals to colonize land. The plants that best succeeded in the Devonian were those with the highest energy efficiencies, which involved size, stability, photosynthesis, internal transport, and reproduction.[240] Plants had different dynamics of extinction than animals did, as plants are more vulnerable to climate change and extinction via competition, but are less vulnerable to mass extinction events than animals.[241]
One of the most important plant innovations was lignin, which is a polymer whose original purpose appears to have been creating tubes for water transport, and was also used to help provide structural support so that trees could grow tall and strong. Without lignin, there would not have been any true forests and probably not much in the way of complex terrestrial ecosystems. Lignin was also responsible for forming the coal beds that powered the early Industrial Revolution, but that coal-bed formation would not happen in earnest until the next geologic period, the aptly named Carboniferous. It took more than a hundred million years for life forms to appear that could digest lignin. A class of fungus gained the ability to digest lignin about 290 mya, and by that time, most of what became Earth's coal deposits had already been buried in sediments.[242] As with other seminal developments in life's history, the ability to digest lignin seems to have evolved only once. The enzyme that fungi use to digest lignin has also been found in some bacteria, but fungi are the primary lignin-digesters on Earth.
From a biomass perspective, the Devonian's primary change was the proliferation of land plants. Below is an artist's conception of the Devonian forests. (Source: Wikimedia Commons)

Land plants comprise about half of Earth's biomass today, and prokaryotes provide the other half. Terrestrial biomass is 500 times greater than marine biomass, and terrestrial plants have about a thousand times the biomass of terrestrial animals, so animals constitute less than 0.1% of Earth's biomass. The ecologies of marine and terrestrial environments are radically different. Virtually all primary producers in marine environments are completely eaten and comprise the food chain's foundation, while less than 20% of land plant biomass is eaten.
Creating the huge biomass of land-based ecosystems meant that carbon was removed from the atmosphere. Also, root systems were a new phenomenon, with dramatic environmental impact. Before the rise of vascular plants, rain on the continents ran to the global ocean in sheets and braided rivers. Every rainfall ran toward the oceans in a flash flood, as happens in deserts today. Plant roots stabilized riverbanks and form the rivers that we are familiar with today. Also, roots broke up rock, accelerated weathering, and created soils. Plants break down rock five times as fast as other geophysical processes will.[243] The forests and soils created a huge "sponge" that absorbed precipitation, which the resultant ecosystems depended on. Vast nutrient runoffs from land into the ocean were stimulated by plants' colonization of land, which in turn stimulated ocean life. The reefs of the Devonian were the greatest in Earth's history and reached about ten times the area of today's reefs, with a total area about equal to half of Europe, of about five million square kilometers (two million square miles).[244]
Plants and trees created a "boundary layer" of relatively calm air near the ground which became the primary abode of most land animals. Also, forests created a positive feedback where moisture was recycled in the forests, and kept them moister than purely ocean-sourced precipitation would. Today, somewhere between 35% to 50% or more of the rain that falls in the Amazon rainforest is recycled water via transpiration.[245] Transpiration also cools the plants via the latent heat of vaporization, as well as the resultant cloud cover.[246] Transpiration, by the way it sucks water from the soils, maintains a negative pressure on soils and keeps them aerated. Waterlogged soils cannot support the vast ecosystems of forest soils, so trees are needed to maintain the soil's dynamics that support the base of the forest ecosystem. Rainforest processes thus create positive feedbacks that maintain the rainforest. Conversely, the rampant deforestation of Earth's rainforests in the past century has created negative feedbacks that further destroyed the rainforests.
Forests were a radical innovation that has not been seen before or since. Trees were Earth's first and last truly gigantic organisms, and the largest trees dwarfed the largest animals. Why did trees grow so large? It seems to be because they could. Land life gave plants opportunities that aquatic life could not provide, and plants "leapt" at the chance. Lignin, first developed for vascular transport, became the equivalent of steel girders in skyscrapers. In the final analysis, trees grew tall to give their foliage the most sunlight and to use wind and height to spread their seeds, and in the future that height would help protect the foliage from ground-based animal browsers. The height limit of Earth's trees is an energy issue: the ability to pump water to the treetops.[247] Arid climates prevent trees from growing tall or even growing at all. Energy availability limits leaf size, too.[248] From an ecosystem's perspective, the great biomass of forests was primarily a huge store of energy; trees allowed for prodigious energy storage per square meter of land. That stored energy ultimately became a vast resource for the forest ecosystem, as it eventually became food for other life forms and the basis for soils, which in turn became sponges to soak up precipitation and recycle it via transpiration. Trees created the entire ecosystem that depended on them.
Revisiting how energy enters ecosystems, primarily via the capture of photon energy by photosynthesis, only so much sunlight reaches Earth, and photosynthesis can only capture so much. The energy "budget" available for plants has constraints, and the question is always what to do with it. An organism can break bonds between atoms and release energy or bind atoms together to build biological structures, which uses energy (exothermic reactions release energy, while endothermic reactions absorb energy). Photosynthesis is endothermic, and in biological systems, endothermic reactions are also called anabolic, as they invest energy to build molecules, which is how organisms grow. Catabolic reactions break down molecules in exothermic reactions that release energy for use. Plants faced the same decisions that societies face today: consumption or investment? Only with an energy surplus can there be investments, such as for infrastructure. Plants invested in trunk-and-branch infrastructure to place their energy-collecting and seed-spreading equipment in the best possible position. Plants race for the sky, and trees represent the biggest energy investment of any type of organism. On average, today's plants use a little more than half of the energy that they capture via photosynthesis (called gross primary production) for respiration. Growing forests use most of that gross primary production to grow (called net primary production), and when the structural limits have been reached, most energy is consumed via respiration to run life processes within the infrastructure.[249] Animal development is similar. When humans began building cities and urban infrastructures, the basic process was the same.
Most marine phyla were unable to manage the transition to land and remain aquatic to this day. Arthropods found a way, and scorpions, spiders, and millipedes were early pioneers. The insect and fish clades comprise the most successful terrestrial animals today, as fish led to all terrestrial vertebrates. Gastropods made it to land, mainly as snails and slugs, as did several worm phyla, but the rest of aquatic life generally remained water-bound. Also, many animal clades have moved back-and-forth between water and land, usually hugging the shoreline, sometimes in a single organism's life cycle, which blurred the terrestrial/aquatic divide at times. The first fish to venture past shore seem to have accomplished it in the mid-Devonian, and colonizing land via freshwater environments was a prominent developmental path.
Although the first insects appeared in today's fossil record about 400 mya, they were fairly developed, which meant that they have an older lineage, probably beginning in the Silurian. The first land animals would have been vegetarians, as something had to start the food chain from plants, and early insects were adapted for plant-eating. Plants would have then begun to co-evolve with animals, as they tried to avoid being eaten.
When life colonized land, global weather systems began dramatically impacting life, as land plants and animals would be at the mercy of the elements as never before, and forests and deserts formed. The continents also began coming together and eventually formed Pangaea in the Permian, and converging plates meant subduction and mountain-building. Mountains in the British Isles and Scandinavia were formed in the Devonian, the Appalachians became larger, and the mountains of the USA's Great Basin also began developing. Colliding tectonic plates can build mountains, and mountain ranges greatly impacted weather systems during terrestrial life's future, which also profoundly influenced oceanic ecosystems.
As with previous critical events, such as saving the oceans and life on Earth itself, life helped terraform Earth. But the late Devonian is an instance when the rise of land plants may have also had Medean effects. Carbon dioxide sequestering, which reduced the atmosphere's carbon dioxide concentration by up to 80%, may have cooled Earth's surface enough so that an ice age began and another one of Earth's mass extinctions began. As with the Ordovician extinction, the ultimate cause for the Devonian extinctions seems to have been rising and falling sea levels, associated with growing and receding ice caps, as Gondwana still covered the South Pole. The Devonian extinction events began happening more than 380 mya, and a major one happened about 375 mya, called the Kellwasser event. The reasons for the Kellwasser event are today generally attributed to the water becoming cold and anoxic.[250] A bolide impact has been invoked in some scientific circles, but the evidence is weak.[251] Mountain-building and volcanic events also happened as continents began colliding to eventually form Pangaea (and the resultant silicate and basaltic weathering removes carbon dioxide from the atmosphere), and those dynamics may have been like what precipitated the previous major mass extinction.[252] Black shales abounded during and after the Kellwasser event, and they are always evidence of anoxic conditions and how the oil deposits initially formed. However, the Kellwasser event anoxia may have not only been due to low atmospheric oxygen, but was also the result of eroding the newly exposed land and the detritus of the new forest biomes, which created a vast nutrient runoff into the oceans that may have initiated huge algal blooms that caused anoxic events near shore.[253]
Unlike the short, severe Ordovician events, the Devonian extinctions may have stretched for up to 25 million years, with periodic pulses of extinction. The Kellwasser event seems to be comprised of several extinction events, and when they ended, at least 70% of all marine species went extinct and the greatest reefs in Earth's history were 99.98% eradicated. It took 100 million years before major reef systems again appeared.[254] Armored fish and jawless fish lost half of their species, and armored fish were rendered entirely extinct in the event that ended the Devonian.
What was most relevant to humans, however, was the almost-complete extinction during the Kellwasser event of the tetrapods that had come ashore. Tetrapods did not reappear in the fossil record until several million years after the Kellwasser event, and has even been referred to as the Fammenian Gap (the Fammenian Age is the Devonian's last age).[255] The Kellwasser event also appeared to be a period of low atmospheric oxygen content, and some evidence is the lack of charcoal in fossil deposits. Recent research has demonstrated that getting wood to burn at oxygen levels of less than 13-15% may be impossible.[256] Because all periods of complex land life show evidence of forest fires, it is today thought that oxygen levels have not dropped below 13-15% since the Devonian, but during the "charcoal gap" of the late Devonian, when the first landlubbing tetrapods went extinct, oxygen levels reached their lowest levels ever, which must have impacted the first animals trying to breathe air instead of water. During the Kellwasser event, there is no charcoal evidence at all, which leads to the notion that oxygen levels may have even dropped below 13%.[257] This drop may be related to severe climatic stresses on the new mono-species forests, which are probably related to the ice age that the forests helped bring about due to their carbon sequestering. That is an attractively explanatory scenario, but the controversy and research continues. The first seed plants probably appeared before the Kellwasser event, but it was not until after the Fammenian Gap that seed plants began to proliferate.[258]
The Kellwasser event ended the first invasion of land by vertebrates and created an evolutionary bottleneck. Some stragglers survived the Kellwasser event, but the fossil record for the next seven million years has been devoid of tetrapod fossils with the exception of one species.[259] After the Fammenian Gap ended about 368 mya, tetrapods renewed their invasion of land, and those tetrapods with many toes appeared in the fossil record with the second invasion. Ichthyostega was Earth's largest land animal in those days. The tetrapods of the time may have not yet been true amphibians, but they were making the adjustments needed to become true land animals, such as losing their gills and improving their locomotion. No new arthropods appeared on land during that time.
After several million years of adaptation, tetrapods seemed ready to become the dominant land animals, but then came the second major Devonian extinction event, today called the Hangenberg event. While the ice age conditions around the Kellwasser event are debated, there is no uncertainty about the Hangenberg event; there were massive, continental ice sheets, accompanied by falling sea levels and anoxic events, as evidenced by huge black shales.[260] The event's frigidity was probably a key extinction factor, and anoxia was the other killing mechanism. The Hangenberg event had devastating consequences; it meant the end of armored fish, the near-extinction of the new ammonoids (perhaps only one genus survived), oceanic eurypterids went extinct, trilobites began to make their exit as seafloor communities were devastated, lobe-finned fish reached their peak influence, and archaeopteris forests collapsed.[261]
Trees first appeared during a plant diversity crisis, and the arrival of seed plants and ferns ended the dominance of the first trees, so the plant crises may have been more about evolutionary experiments than environmental conditions, although a carbon dioxide crash and ice age conditions would have impacted photosynthesizers. The earliest woody plants that gave rise to trees and seed plants largely went extinct at the Devonian's end. But what might have been the most dramatic extinction, as far as humans are concerned, was the impact on land vertebrates. During the Devonian extinction about 20% of all families, 50% of all genera, and 70% of all species disappeared forever.
There seems to have been convergent evolution among the early tetrapods, but they were beaten back twice during the late- and end-Devonian extinction events, and what emerged the third time was different than what came before.[262] As with many mass extinction events, evolution's course was significantly altered in the extinction's aftermath. As with studies of human history, events are always contingent and not foreordained in Whiggish fashion. Although the increase in "intelligence" may well be an inherent purpose of being in physical reality, the evolutionary path to the man writing these words had false starts, "detours," singular events, expansions, bottlenecks, catastrophes, and the like. Evolutionary experiments on other planets probably had radically different outcomes. A mystical source that I respect once stated that there are one million sentient species in our galaxy, with a diversity that is staggering, and from what I have been exposed to (and here), I will not challenge it.

Making Coal, the Rise of Reptiles, and the Greatest Extinction Ever

World map in early Carboniferous Period (c. 340 mya) (Source: Wikimedia Commons) (map with names is here)

World map at end of Carboniferous Period (c. 300 mya) (Source: Wikimedia Commons) (map with names is here)

World map in late Permian Period (c. 260 mya) (Source: Wikimedia Commons) (map with names is here)

Chapter summary:

• Vertebrates resume their invasion of land after 14 million years
  
  
• Structural respiration limitation for amphibians and early reptiles
  
  
• Oxygen levels rise to highest ever on Earth, caused by first rainforests
  
  
• Giant arthropods appear
  
  
• Amniotes appear
  
  
• Sharks and ray-finned fish rise to dominance
  
  
• Ice age leads to Carboniferous rainforest collapse, ending greatest period of coal formation in Earth's history
  
  
• Lignin digesting white rot appears, ending possibility of a comparable coal-building period happening again
  
  
• Tethys Ocean begins forming in the Permian
  
  
• Conifer forests appear
  
  
• Pangaea supercontinent forms
  
  
• Synapsids dominate terrestrial biomes
  
  
• Early instance of display features for sexual selection
  
  
• Reptiles develop more upright posture, improving breathing, while oxygen levels crash
  
  
• First extinction event of Permian extinction
  
  
• Volcanism ends ice age and initiates 200 million year Greenhouse Earth phase
  
  
• Animals adapt to low oxygen levels
  
  
• Mammalian ancestors appear
  
  
• Reefs begin recovering from Devonian extinction
  
  
• Greatest extinction event ever
  

The period succeeding the Devonian is called the Carboniferous (c. 359 to 299 mya), for reasons that will become evident. The Hangenberg event cut short the second attempt of vertebrates to invade land and there was another 14-million-year gap in the fossil record called the Tournaisian Gap, which is part of Romer's Gap (which is considered to be about a 30-million-year gap).[263] After all mass extinctions, it took millions of years for ecosystems to recover, even tens of millions of years, and markedly different ecosystems and plant/animal assemblages often replaced what existed before the extinction. The Devonian spore-forests were destroyed, and outside of the peat swamps, the tallest trees in the Tournaisian Gap were about as tall as I am, and even in the swamps, the tallest trees were about ten meters tall, as they were before the Hangenberg event.[264]
Peter Ward led an effort to catalog the fossil record before and after Romer's Gap, which found a dramatic halt in tetrapod and arthropod colonization that did not resume until about 340-330 mya. Romer's Gap seems to have coincided with low-oxygen levels of the late Devonian and early Carboniferous.[265] If low oxygen coincided with a halt in colonization, just as the adaptation to breathing air was beginning, the obvious implication is that low oxygen levels hampered early land animals. Not just the lung had to evolve for the up-and-coming amphibians, but the entire chest cavity had to evolve to expand and contract while also allowing for a new mode of locomotion. When amphibians and splay-footed reptiles run, they cannot breathe, as their mechanics of locomotion prevent running and breathing at the same time. Even walking and breathing is generally difficult. This means that they cannot perform any endurance locomotion, but have to move in short spurts. This is why today's predatory amphibians and reptiles are ambush predators. They can only move in short bursts, and then have to stop, breathe, and recover their oxygen deficit. In short, they have no stamina. This limitation is called Carrier's Constraint. The below image shows the evolutionary adaptations that led to overcoming Carrier's Constraint. Dinosaurs overcame it first, and it probably was related to their dominance and the extinction or marginalization of their competitors. (Source: Wikimedia Commons)

The heart became steadily more complex during complex life's evolutionary journeys. Fish hearts have one pump and two chambers. Amphibians developed three-chambered hearts, wherein oxygenated and deoxygenated blood are not structurally separated, but mix. That arrangement is obviously not as energy-efficient as separating oxygenated and deoxygenated blood. Some later reptiles evolved four-chambered hearts, which their surviving descendants, crocodilians and birds, possess, and somewhere along the line, mammals also evolved four-chambered hearts, perhaps before they became mammals.
While oxygen level changes of the GEOCARBSULF model show early fluctuations that the COPSE model does not, both models agree on a huge rise in oxygen levels in the late Devonian and Carboniferous, in tandem with collapsing carbon dioxide levels. There is also virtually universal agreement that that situation is due to rainforest development. Rainforests dominated the Carboniferous Period. If the Devonian could be considered terrestrial life's Cambrian Explosion, then the Carboniferous was its Ordovician. In the Devonian, plants developed vascular systems, photosynthetic foliage, seeds, roots, and bark, and true forests first appeared. Those basics remain unchanged to this day, but in the Carboniferous there was great diversification within those body plans, and Carboniferous plants formed the foundation for the first complex land-based ecosystems. Ever since the Snowball Earth episodes, there has almost always been a continent at or near the South Pole, and the [url=http://www.ahealedplanet.net/humanity.htm#ice