Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe

by Lisa Randall

The second extinction pulse was probably due to a later warming period that wiped out the fauna that had adapted to the cold. Warm-adapted fauna such as tropical plankton, shallow-water crinoids (predecessors of starfish and sea urchins), trilobites, armored fish, and coral were the first to go, then the cold-adapted versions of coral, trilobites, and brachiopods went after.

The next mass extinction lasted a while—about 20 million years—and started about 380 million years ago in the late Devonian period near the Devonian-Carboniferous transition. It seems there were a number—the number is uncertain, but suggestions vary from three to seven—of pulsed extinctions, each lasting about a few million years. This event hit marine life hard too, killing a significant fraction of species that lived in the oceans. Insects, plants, and early proto-amphibians survived on land, though extinctions were rampant there as well.

The Permian-Triassic (P-Tr) event about 250 million years ago was the most devastating known extinction in terms of the percentage of species that disappeared from the planet. Life, including amphibians and reptiles, had blossomed both in the sea and on land for quite some time after the Devonian extinction. But that ended during the P-Tr extinction, when at least 90 percent and probably more of the species on both land and in the sea died off. The losses included surface plankton as well as bottom-dwelling species such as fixed bryozoans and corals, some shellfish, and trilobites—species that had already survived two major extinction events. On land, even insects were devastated—the only time they suffered a mass extinction. In addition, a large portion of amphibians disappeared, and reptiles—who had only emerged after the previous extinction—lost most of their member species too. The cause of the P-Tr extinction remains the subject of controversy, but massive climate change and changes in the chemistry of the atmosphere and oceans almost certainly played a role. Though the cause and mechanism are unclear, the approximately eight degree Celsius rise in temperature was likely due at least in part to massive volcanism in Siberia and the consequent enormous amount of carbon dioxide and methane emissions from the Siberian Traps. The Permian-Triassic extinction—the biggest known extinction event—is almost certainly partly the result of these volcano-induced gases that heated t...

But this blossoming of life was interrupted within 40 or 50 million years by the fourth mass extinction, which arrived about 200 million years ago. Roughly 75 percent of all species went extinct in this end-Triassic extinction that preceded the Jurassic period. The cause of this extinction is uncertain, but lower sea levels and the start of the volcanic rift that eventually produced the Atlantic Ocean might have played a role. Most large vertebrate predators in the ocean were killed off and species of sponges, corals, brachiopods, nautiloids, and ammonoids were badly hit as well. This extinction also eliminated most mammal-like creatures, many large amphibians, and nondinosaur archosaurs.

The removal of any real competition on land left the dinosaurs essentially in charge. Extinctions destroy life, but they also reset the conditions for life’s evolution. The Jurassic period that followed is famous for the book and movies named for it, even if not all the creatures featured in Jurassic Park lived during that interval. But the Jurassic period is indeed the time when dinosaurs rose to prominence—achieving dominance over the ecosystem on land by the late Jurassic. Flying reptiles, crocodiles, turtles, and lizards multiplied in this period and mammals evolved too, though their days in the limelight would have to await the mass extinction that would follow.

The most recent mass extinction is probably also the most famous. It is the one that occurred at the boundary between the Cretaceous and Paleogene periods. This event, formerly known as the K-T extinction (which stood for Cretaceous-Tertiary) but now officially known as the K-Pg extinction (because the Tertiary Period was renamed the Paleogene), occurred 66 million years ago. It is most commonly known as the one that killed off the dinosaurs. But dinosaurs weren’t the only species to go extinct. About three-quarters of the species and half the genera alive at the time disappeared, including many reptiles, mammals, plants, and marine life. ...

Following the K-Pg extinction, mammals became a much bigger player on Earth. Though many factors contributed, the elimination of the terrestrial dinosaurs was almost certainly relevant. Large mammals (such as ourselves) might never have risen to prominence had terrestrial dinosaurs who dominated the competition for essential resources not first been eliminated. One speculation for why dinosaurs fared so much better than mammals before the Chicxulub impact is that dinosaurs laid eggs in volume, whereas mammals have fewer offspring and give birth less frequently the larger they are. Dinosaurs might have simply won a numbers game in their competition with large mammals.

Dinosaurs lived in the Mesozoic era, which ranged from 252 to 66 million years ago. (See Figure 29.) The name Mesozoic comes from the Greek term for “middle life” and indeed this era lies in the middle of the three geological eras of the Phanerozoic

Keep in mind that the term organic in chemistry simply refers to the presence of carbon, not necessarily to elements of life. The term, though unfortunate, is of course not coincidental in that some (but not all) organic molecules are essential to life as we know it.

Whether or not greenhouse gases fully accounted for the Earth’s warmer-than-expected early temperature, liquid oceans were fairly clearly present in the early Earth. So one or more of the above resolutions must have played a role.

On Earth, the Sun is undoubtedly the chief source of energy. The power from sunlight today is thousands of times greater than that of the next most significant source, geothermal heat.

Based solely on the Earth’s reflectivity and the Sun’s luminosity and distance from us, water on the Earth’s surface would be frozen even today without the atmosphere’s warming effect. Although we legitimately worry about too much heating in today’s atmosphere, the Earth would be too cold without the greenhouse effect of carbon dioxide, methane, water vapor, and nitrous oxide that keeps it warmer. Liquid water currently exists on Earth only because of these greenhouse gases, which absorb infrared light and warm the planet, thereby establishing equilibrium.

The habitable zone is the region where conditions are such that life can survive. It is the “Goldilocks” region that is just right to allow for stable liquid water. Too far away from the major heat source—the Sun—and water will be ice. Too close in and the water won’t condense onto a planet’s surface in the first place. Water might exist below the surface of a planet too, though that is unlikely to house the diversity of life that a large ocean can promote.

In another four billion years or so, the Sun will turn into a red giant, and a few billion years after that, it will burn out completely. According to current models, no forms of Earth-bound life—simple or complex—will survive in that distant future.

A planet hosting life has to be far enough away from the Sun to avoid excessive solar radiation but perhaps should be sufficiently close to be protected by outer planets from asteroids. Whether or not this is necessary, Jupiter certainly does play the role of the Earth’s big brother—or bouncer—protecting its smaller “sibling” from extraterrestrial attacks and making the development of life that much simpler.

Such deposits have been critically important for the modern era too. A lot of the gold, tungsten, nickel, and other valuable elements in the Earth’s crust are accessible because of extraterrestrial objects that pelted the Earth.

Comets have the further feature that they carry a disproportionate amount of energy compared to asteroids, since they are generally moving at faster speeds—up to 70 or more km/sec as opposed to 10 to 30 km/sec for asteroids. Typically, a ballistic missile travels at less than 11 km/sec, an asteroid at about 20 km/sec, a short-period comet at more like 35 km/sec, and a long-period comet at 55 km/sec, though faster speeds occur too. (See Figure 32.) Kinetic energy grows not only with mass, but also with the square of the velocity. Comets’ greater speeds mean that even less frequent comet impacts, or ones from smaller objects, could in principle do greater damage than slower-moving asteroids.

Recall that the Milky Way is a disk galaxy, meaning most of the stars and gas lie in a thin disk, about 130,000 light-years across but only roughly 2,000 light-years in thickness. The Sun is located at a distance of about 27,000 light-years from the galactic center, and happens at this moment to be close to the galactic midplane—less than 100 light-years away. It is also at the edge of a spiral arm.