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June 21 - June 27, 2022
At the dawn of the universe, matter consisted of quarks, leptons, and gluons, a curious cast of subatomic particles that would eventually coalesce into atoms.
something that interacts with normal matter through gravity but doesn’t interact with light; astronomers dubbed it dark matter.
Together, dark matter and dark energy are thought to make up some 95 percent of all that exists, enigmatic constituents that we can’t detect but which are thought to have played a major role in shaping the universe.
Our planet coalesced some 4.54 billion years ago, but Earth’s oldest known rocks date back only to about 4 billion years. Older rocks must have existed, but they’ve been eroded away or were buried and transformed through metamorphism into unrecognizable form. A few may still lie in some remote Canadian or Siberian hillside, waiting to be recognized, but largely, the first 600 million years of Earth history constitutes our planet’s Dark Age.
The inner core—a ball with a radius of 762 miles (1,226 kilometers)—is solid, while the outer core (some 1,475 miles, or 2,260 kilometers thick) remains molten and slowly moves by convection, as dense material near the base heats up and begins to rise, eventually to cool and sink back down toward the base. This motion of the outer core generates an electrical dynamo, resulting in Earth’s magnetic field.
About two-thirds of our planet’s mass, the mantle consists mainly of silicate minerals—minerals rich in silicon dioxide (SiO2—quartz in its pure crystalline form) joined by magnesium and lesser amounts of iron, calcium, and aluminum.
Less than 1 percent of our planet’s mass, the crust—the thin shell in our egg analogy—is the only layer we can observe and sample routinely, providing a remarkable trove of knowledge. Continents are made of crust containing quartz (SiO2) and feldspar minerals rich in sodium or potassium,
As heat wicked away to the atmosphere, the magma ocean soon cooled to form a primordial crust of broadly basaltic composition. And as this crust thickened and began to melt from its base, silica-rich rocks broadly similar to granite began to form—the first continental crust.
Uranium-238 has a half-life of 4.47 billion years, meaning that on this timescale half of the uranium-238 in a sample will have decayed to lead-206; similarly, uranium-235 has a half-life of 710 million years. Because no lead entered the zircons as they formed, any lead we measure in them today must have formed by the radioactive decay of uranium.
Zircons, then, suggest that the differentiation of Earth’s crust began early in our planet’s history. The chemistry of oxygen in the zircons also suggests that liquid water was already present 4.38 billion years ago; Earth’s hydrosphere is nearly as old as the planet.
Chondritic meteorites, then, provide a source of water and carbon, and unlike comets, they pass the hydrogen isotope test. Thus it looks like chondritic meteorites of different kinds provided most of the rock, water, and air that we call home.
over the past 100 million years, the Atlantic Ocean has widened by nearly 1,600 miles
it is the sinking of crustal slabs that pulls the ocean crust apart; new crust then forms passively at the ridges. As subducting slabs sink into the hot mantle, they begin to melt, generating volcanoes as molten material rises to the surface.
subduction has destroyed most oceanic crust older than 180 million years.
Life, then, is characterized by growth and reproduction, metabolism, and evolution.
The last common ancestor of all organisms alive today must have approximated the cells of bacteria, but even the simplest bacteria are complicated molecular machines, a product of evolution, not its starting point.
Iron formations are distributed widely in sedimentary basins older than about 2.4 billion years, but fall off markedly after that time, suggesting that this was when O2 began to permeate the atmosphere and surface ocean.
when we examine sandstones deposited along coastlines before 2.4 billion years ago, we find grains of pyrite that were eroded from a source on land, carried downstream by rivers, and, finally, deposited along the edge of the sea—all without coming into prolonged contact with even small amounts of oxygen. In sedimentary successions younger than 2.4 billion years, we rarely see such grains.
prior to 2.4 billion years ago, chemical processes in the atmosphere played a major role in Earth’s sulfur cycle in a way that ceased after that time. Chemical models suggest that this telltale isotopic signature can only be imparted when oxygen levels in the atmosphere are extremely low—less than 1/100,000th of today’s abundance.
Turning carbon dioxide into sugar requires electrons, which plants and algae extract from water, generating O2 in the process. This carries a high energy cost, but when the environment is oxygen-rich there aren’t any alternatives. Where light is present but O2 absent, however, other sources of electrons become available: hydrogen gas, hydrogen sulfide with its rotten egg smell, and iron ions in solution, among others.
On the early Earth, diverse bacteria and archaea populated land and sea, cycling carbon, iron, sulfur, and other elements. More complicated organisms—algae, protozoans, fungi, plants, and animals—require oxygen for metabolism and so would have to wait in the evolutionary wings until O2 became a persistent component of the Earth’s surface.
oxygen in the air we breathe owes its existence to life. The only process capable of oxygenating our planet’s atmosphere is oxygenic photosynthesis—photosynthesis in which water supplies electrons, generating O2 as a by-product.
Today, rates of photosynthesis are generally limited not by sunlight, carbon dioxide, or water, but rather by the availability of nutrients, especially phosphorus, found in DNA, membranes and ATP, the cell’s energy currency, and nitrogen, required for both DNA and proteins.
As our planet matured, large, stable continents emerged above sea level, increasing the erosional flux of phosphorus to the oceans. Eventually, as the phosphorus supply came to outstrip the availability of alternative electron donors, cyanobacteria gained ecological importance. And as they did so, they transformed the world. The oxygen they produced scavenged other sources of electrons from sunlit waters, permanently tipping the biosphere toward oxygenic photosynthesis and oxygen-rich air.
eukaryotes, unlike bacteria, have a dynamic internal system of molecular scaffolding and membranes that allows their cells to grow large and take many different shapes. It also enables eukaryotes to make a living in ways that bacteria generally can’t, in particular by engulfing small food particles, including other cells. Through predation, then, eukaryotic cells brought new complexity to ecosystems.
It increasingly looks like the eukaryotic cell itself emerged from a long-ago partnership between an archaean cell and a bacterium capable of aerobic respiration.
we begin to see fossils of eukaryotic cells in sedimentary rocks deposited 1.6–1.8 billion years ago.
Even today, the biosphere has 30 tons of bacteria and archaea for every ton of animal.
In uppermost Proterozoic rocks, deposited on the heels of a vast global ice age, large complex organisms appear in the fossil record. More than three billion years after life emerged, the age of animals was at hand.
The Cambrian Period (541–485 million years ago) is famous as the interval during which we first see abundant fossils of familiar-looking animals. The conventional record of Cambrian animals, recorded by mineralized shells and other skeletons, is dominated by extinct arthropods called trilobites; these segmented, multilimbed creatures comprise some 75 percent of all fossil species discovered in Cambrian rocks.
Cambrian limestones still formed mostly by physical or microbially facilitated calcium carbonate precipitation. (Today, skeletons account for most limestone deposition in the oceans.)
Today, some 400,000 species of land plant account for half of Earth’s photosynthesis and an estimated 80 percent of our planet’s total biomass.
Animals, although born in ancient oceans, are today most diverse on land—insect species alone far outnumber all animal species in the sea.
The earliest known true mammals, turtles, lizards, and frogs all occur in rocks of the Triassic Period (252–201 million years ago), along with pterosaurs (the first winged vertebrates), dinosauromorphs (the earliest true dinosaurs and their close relatives), and other now-extinct groups.
Where we do find common ground is that in each case, environmental disruption was rapid; the rate of environmental change was as important as its magnitude. When environmental change is slow, populations can adapt to their shifting circumstances, but when it is fast, adaptation may be challenging, leaving migration or extinction as the only available options.
The modern world is full of mammals in part because dinosaurs became extinct. Fish in the open ocean radiated only after end-Cretaceous mass extinction eliminated the ammonites.
Rising mountains increased weathering rates, pulling carbon dioxide from the atmosphere, while shifting plates reoriented seawater circulation in the oceans. In consequence, Earth eventually began to cool.
Called hominins, these new apes were broadly chimp-like: small in stature, with a small brain, protruding snout, and long arms with elongate curving fingers to facilitate movement through tree tops. But the hominins differed from other great apes in one critical way. They could walk upright.
H. erectus is notable for two reasons. First, its anatomy is intermediate between those of australopithecines and modern humans, with a more human-like skeleton and a brain larger than Lucy’s but smaller than ours. And second, unlike all earlier hominins, H. erectus prospered not only in Africa but throughout Eurasia as well.
The oldest fossils assigned to Homo sapiens occur in 300,000-year-old rocks from Morocco. These fossils are only a bit younger than evidence for a new and sophisticated tool-making culture and the widespread (and controlled) use of fire.
Early humans lived only in Africa, but a bit more than 100,000 years ago, one population stuck its toe into the wider world, dwelling in what is now Israel along with Neanderthals. Then, 50,000–70,000 years ago, our species spread rapidly throughout Asia and Europe.
Recently, archaeologists reported evidence that humans lived along the Salmon River in Idaho 16,500 to 16,300 years ago, recording a first wave of migration from northeastern Asia, probably along the Pacific coast.
A second and ultimately decisive influence began about 11,000 years ago in a crescent that curves northward from Israel and Jordan to Syria, Turkey, and Iraq. Here people first developed agriculture, learning to grow and harvest figs, barley, chickpeas, and lentils. Within a thousand years, sheep, goats, pigs, and cattle had been domesticated.
In the physical part of the carbon cycle, volcanoes add CO2 to the atmosphere, while chemical weathering removes it, the carbon eventually depositing as limestone.
