So far, this chapter has concentrated on the evolution of our ancestors. Of course, they did not live in a vacuum. They were influenced by major themes of change in the world around them. They constantly interacted with other life forms and the physical environment of the planet itself. Here are some of the major headlines of the past few billion years that played a part in our back story.
C. Life on Land
Continents and ocean floors are much more than a random map of high- and low-lying lands. They are made of different forms of rock and have vastly different histories. Earth has always had oceanic crust. It is thinner and denser, formed mostly of basalt. Islands and continents are mostly formed from granite. Continental crust came later in geologic history, yet today’s continents are older than today’s seafloor. Ocean crust is continuously created out of the Earth’s deeper mantle and recycled back down into it, in the process of plate tectonics. Continental crust is “unsinkable” and continues to accumulate on the surface. All dry land is continental crust, but not vice versa; some continental crust extends into shallow seas too. The amount of exposed land depends on sea level.
When Earth was young, there was not yet enough granite crust to form true continents. The worldwide ocean was dotted with island arcs. Then some of the islands got pushed together by plate tectonics to form larger land masses. This process helped create even more new land. When ocean crust subducts into the mantle beneath dry land, it sometimes partially melts. The resulting magma (liquid rock) is granite, less dense than the solid basalt, so it rises to the surface and hardens to create more continental crust. This very gradual process formed the continental bedrock or shields.
In the time scale of the last few billion years, continents have grown further by the processes of mountain-building (orogeny) and sedimentation. Mountains are formed when two landmasses slam together in slow motion. As the land at their margins is compressed together, it folds vertically and can be raised well above sea level. As mountains are worn down by water, weather and gravity, the silt gets washed down into plains and coastal waters, where it hardens into flat sedimentary rock. Layer by layer, this platform rock has expanded and thickened the low-lying continental crust by hundreds of meters, thereby increasing the amount of exposed dry land. Sedimentary rock has also played an important role by preserving the imprints of some living things as fossils.
Through the processes of plate tectonics, orogeny, and sedimentation, the continents grew to sizable land masses by 2.5 billion years ago. 2 The globe at that time still did not resemble the one we know today. Continents had unfamiliar shapes, and they were not in their present locations. In fact, on the time scale of a billion years, a continent can drift around the entire world more than once.
Even though life was predominantly aquatic before 500 MYA, dry land had a major impact on the biosphere well before that. Continents created an entirely new global climate, affecting the absorption of sunlight, the formation of clouds, the accumulation of greenhouse gases, and the flow of wind and water. Furthermore, most aquatic plants and animals thrived in shallow waters along the continental shorelines. With growing continents, their habitat grew ever larger. In return, living things had an impact on the land too. The ingredients of Earth’s crust multiplied from just a few simple minerals to thousands of complex minerals. Part of this diversification was due to geothermal energy, but most of the minerals required the oxygen that was being released by photosynthetic life. As far as we know, gems such as turquoise and malachite exist nowhere else in the universe. 3
A few centuries ago, paleontologists were baffled that the fossil record began very abruptly at a particular layer of sedimentary rock. The oldest fossils were dated to about 540 MYA; many sources even quote this age as precisely as 543 million years. We now understand that these Cambrian 1 fossils were not the first animals, but simply the oldest ones that are still well preserved. They were the first macroscopic animals with hard body parts – teeth, scales, bones, and shells. Mineralized tissues fossilize much more readily than soft parts like skin and organs. Seashells evolved so quickly and numerously that their appearance is heralded as the Cambrian “explosion”.
There are numerous proposed causes for the explosion, including an upswing of oxygen 4 and a wealth of minerals from continental erosion. 5 Another important consideration is that the evolution of hard body parts was a positive feedback loop, a process that reinforces itself. The animals with shells had protection from predators, so they outlasted their naked soft-body neighbors. The predators with teeth were able to crack open some shells, so they survived better than their soft-gummed peers. Bigger teeth created evolutionary pressure toward harder shells, which created pressure toward sharper teeth. We descend from the predators of the Cambrian age, the animals with teeth. Our earliest toothed ancestors were bottom-dwelling jawless fish like lampreys.
As an important tool for this hunting game, eyes became common during the Cambrian explosion. Vision provided obvious survival advantages for predators and prey alike. Eyes evolved independently in many different species and phyla around the time of the Cambrian explosion, derived from light-sensitive cells inherited from an early animal common ancestor. 6 Visual processing requires heavy nerve activity. Lumps of nerves that were involved in organ control, motion, and vision began to clump together into brains.
The Cambrian explosion was truly important, not only for life on Earth but for those who study it. It was a worldwide surge of evolutionary diversity and the beginning of mineral body parts. It also bestowed scientists with the gift of anatomical fossils, one of the most valuable records of Earth’s natural history. Even so, histories of discovery tend to overstate this event. To the unaided human eye, the Cambrian explosion used to look like the beginning of life, or at least the speciation of all the animal phyla. In truth, there was plenty of pre-Cambrian animal life, representing several phyla. 7 The Cambrian explosion just made them easier for us to find. As for our ancestors, it was a period of gradual evolution like any other.
Though they get all the glory, our four-legged forefathers were far from the first life on land. In fact, vertebrates were just about the last phylum to come out of the water. The first tetrapods would not have been so tempted to come ashore without the paradise that already awaited them. As would be expected, bacteria beat everyone at this race, and have been colonizing dry land ever since it first appeared 3 BYA. 8 Plants, fungi, and invertebrates all started to spread ashore about 500 MYA, 100 MY before tetrapods.
There were obvious advantages and disadvantages for plants relocating from sea to land. They gained exposure to sunlight at the expense of access to water. The first land plants were coverings like moss and algae that clung to wet rocks. To advance further inland, they had to develop a waxy covering to retain water, as a well as roots and a vascular system to transport food and water throughout the organism. At the time amphibians came ashore, plant life was still primitive. There were spiny shrubs and small vascular plants with stalks and spores. Further land-plant adaptations like wood, leaves, and seeds evolved in the next 100 MY. To our eyes, the most striking feature of the Carboniferous landscape would have been the absence of flowers and grass.
Fungi and plants seem to have helped each other make the landward transition. 9Lichen, a symbiotic mat of algae and fungus, is usually one of the first “pioneer organisms” to invade new volcanic islands. We can assume it was one of the first eukaryotic colonists on land half a billion years ago. A fossilized organism called prototaxites is believed to be a fungus five meters tall and a meter wide. In a world before trees, this “humongous fungus” 10 was the towering feature on the skyline! 11
Arthropods (shrimp, lobsters, crabs) enjoyed great seafloor success in the Ordovician period, and the transition to land was easier for them than for vertebrates. Arthropods had an exoskeleton to support their weight and protect them from sun and dehydration. The arthropods that became adapted to terrestrial life 450 MYA speciated into what we now call “bugs” – insects, spiders, and the like. Millipedes are the oldest known air-breathing animal. 12 It wasn’t long before the air was abuzz with flying insects, one of evolution’s greatest success stories. 13 Other creepy crawlies such as snails and slugs also invaded land before or concurrently with vertebrates. 14
Bacteria, fungi, decaying plant matter, and burrowing bugs teamed up to prime the land in a groundbreaking way. 2 They turned barren sand into fertile soil. Soil is rich in nitrogen, potassium, and phosphorus, which plants need to supplement photosynthesis. Nitrogen is an especially interesting part of the cycle. Though it is abundant in the air, it is locked up in very strong triple bonds that a plant cannot break down. Plants depend on nitrogen-fixing bacteria to perform this metabolic step.
By 360 MYA, dry land boasted full ecosystems, self-supporting communities of life. 15 Rich soil spurred the evolution of ever-larger plants until continents were full of lush forests. Tetrapods sat at the top of the food chain. They enjoyed a banquet of plants and insects, with no predatory sea creatures chasing after them. It’s no wonder they stepped up out of the river!
Forests changed the world. They provided another major source of atmospheric oxygen. In the Carboniferous period, oxygen reached its all-time highest level of concentration. This made the atmosphere dense and high in energy, factors that led to the period’s famously giant insects like 18-inch dragonflies. Plants are a carbon-dioxide sink, so they also reduced the atmosphere’s greenhouse effect. This was especially a problem for the world’s first trees, because other organisms had not yet evolved the ability to decompose bark and release the carbon back into the atmosphere. 16 It turned out to be suicidal. The planet cooled into an ice age that killed off a good fraction of the swampy forests.
Living things are formed primarily of the four elements carbon, hydrogen, oxygen, and nitrogen. Normally at death, they completely decompose and the elements are recycled back into the soil, water, or air. When organisms happen to die underwater in a low-oxygen environment, they do not decompose quickly but accumulate as bottom sludge. The organic matter gets buried under rock and gets compressed, heated, and processed over millions of years until only the hydrogen and carbon remain. These hydrocarbons have high-energy bonds and are what we extract today as fossil fuels.
Natural gas or methane, CH4 , has a long history predating the Cambrian explosion. Most reserves have formed from marine microorganisms settling to the ocean floor. 17 Some bacteria and archaea have been giving off natural gas as a waste product for billions of years. In fact, some natural gas even occurs, well, naturally, without a biotic source. The largest reserves of natural gas are found in present-day Russia, Southwest Asia, and the US.
Most coal is a byproduct of the Carboniferous forests of 300 – 360 MYA, especially in present day Europe and the Eastern US. Some natural gas was produced along with coal. After the Carboniferous, fungi evolved to decompose tough tree bark. Rotting bark helped restore the carbon cycle, making it much more difficult for woodlands to get locked away underground in coal reserves anymore.
Mankind has come to take fossil fuels for granted, but they are actually a unique lucky gift from one specific era to another. They are known as non-renewable energy resources because Earth’s store of them will be depleted in this millennium. The race is on to harvest renewable energy sources like the sun, or to perhaps eventually mine natural gas from other worlds.
- Carboniferous forest sketch by unknown illustrator for books Our Native Ferns and their Allies (Underwood, 1896 ed) and / or Manual of Geology (Dana, 1874). https://commons.wikimedia.org/wiki/File:Our_Native_Ferns_-_Carboniferous_Pteridophyta.jpg (accessed and saved 7/02/15) ↩
- “Algol”, “Interactive Continental Drift”, YouTube (12/26/2018), https://www.youtube.com/watch?v=UgRHZ5jDPUU (accessed and saved 8/17/19). ↩
- Robert M. Hazen et al., “Mineral evolution”, American Mineralogist 93(11-12):1693-1720 (11/01/2008), https://pubs.geoscienceworld.org/msa/ammin/article-abstract/93/11-12/1693/44643 (accessed 8/18/19). ↩
- Tianchen He et al., “Possible links between extreme oxygen perturbations and the Cambrian radiation of animals”, Nature Geoscience 12, 468-474 (5/06/2019), https://www.nature.com/articles/s41561-019-0357-z (abstract accessed 8/18/19). ↩
- Shanan E. Peters and Robert R. Gaines, “Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion”, Nature 484, 363-366 (4/19/2012), https://www.nature.com/articles/nature10969 (abstract accessed 8/18/19). ↩
- Dan-E. Nilsson, “Eye ancestry: Old genes for new eyes”, Current Biology 6(1): 39–42 (Jan. 1996), http://www.sciencedirect.com/science/article/pii/S0960982202004177 (accessed and saved 8/18/19). ↩
- See e.g. Daniel Y.-C. Wang, Sudhir Kumar, and S. Blair Hedges, “Divergence Time Estimates for the Early History of Animal Phyla and the Origin of Plants, Animals and Fungi”, Proceedings of the Royal Society B: Biological Sciences 266(1415):163-171 (1/22/1999), http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1689654/ (accessed 8/18/19). ↩
- Fabio U. Battistuzzi, Andreia Feijao, and S. Blair Hedges, “A genomic timescale of prokaryote evolution: insights into the origin of methanogenesis, phototrophy, and the colonization of land”, BMC Evolutionary Biology, 4:44 (11/09/2004) , http://www.ncbi.nlm.nih.gov/pmc/articles/PMC533871/ (accessed and saved 8/18/19). ↩
- Claire P. Humphreys et al., “Mutualistic mycorrhiza-like symbiosis in the most ancient group of land plants”, Nature Communications 1, article 103 (11/02/2010), http://www.nature.com/ncomms/journal/v1/n8/full/ncomms1105.html (accessed and saved 8/18/19). ↩
- As far as I can trace it, Stephen Jay Gould was the first to use this phrase in his essay, “A humongous fungus among us”, Natural History 101(7):10-18 (July, 2002), http://digitallibrary.amnh.org/handle/2246/6498 (pp. 542-550 of PDF) (accessed and saved 8/18/19). ↩
- C. Kevin Boyce et al., “Devonian landscape heterogeneity recorded by a giant fungus”, Geology 35(5):399 – 402 (5/01/2007), http://geology.gsapubs.org/content/35/5/399.abstract (accessed and saved 8/18/19). ↩
- Paul Selden and Helen J. Read, “The oldest land animals: Silurian millipedes from Scotland”, Bulletin of the British Myriapod & Isopod Group 23: 36 – 37 (2008), http://www.bmig.org.uk/content/bmig-bulletin-volume-23-2008 (accessed and saved 8/18/19). ↩
- Bernhard Misof et al., “Phylogenomics resolves the timing and pattern of insect evolution”, Science 346(6210):763-767 (11/07/2014), https://science.sciencemag.org/content/346/6210/763 (accessed and saved 8/18/19). ↩
- Paul Bunje, “The Gastropoda”, California Academy of Sciences (1999), https://ucmp.berkeley.edu/taxa/inverts/mollusca/gastropoda.php (accessed and saved 8/18/19). ↩
- William A. Shear, “The early development of terrestrial ecosystems”, Nature 351, 283-289 at 285 (5/23/1991), https://www.nature.com/articles/351283a0 (accessed and saved 8/18/19). ↩
- Dimitrios Floudas et al., “The Paleozoic Origin of Enzymatic Lignin Decomposition Reconstructed from 31 Fungal Genomes”, Science 336 (6089): 1715–1719 (6/29/2012), https://science.sciencemag.org/content/336/6089/1715.long (accessed and saved 8/18/19). ↩
- “How natural gas is formed”, Union of Concerned Scientists (4/03/2015), http://www.ucsusa.org/clean_energy/our-energy-choices/coal-and-other-fossil-fuels/how-is-natural-gas-formed.html#.VXam2MLbLcs (accessed and saved 8/18/19). ↩
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