The Elements of Evolution – Life History Synopsis

Life History Synopsis

▫ What primarily defines life is the will to live. Life blossomed into a plethora of forms, each vectored by adaptive impulses to the immediate environment. As life diversified, interdependence among species intensified in both competitive and cooperative relationships.

Geological Time

▫ Earth’s 4.55-billion-year history is divided into a nested hierarchy of eons, eras, periods, epochs, and ages: eons have eras, eras comprise periods, and so on.

▫ Each of the 4 eons are characterized by major life stages: Hadean (4.54–3.8 BYA): before life; Archean (3.8–2.5 BYA): before an oxygenated atmosphere; Proterozoic (2,500–542 MYA): early life; and Phanerozoic (542 MYA–now): life that leaves lots of fossils.

▫ The end of each geological era is marked by a major mass extinction event.

Mass Extinctions

▫ The episodic adventures of organisms have been periodically punctuated by extinction events that punked life’s prospects.

▫ Mass extinction events have invoked dramatic changes in dominant life forms. Mass extinction events ranging over a spectrum of intensity have been episodic in Earth’s history.

Mass extinctions are largely indiscriminate. The only selection factor in extinction events is tolerance to adversity. The hardiest organisms are the most archaic: microbes, but they too go extinct.

▫ High extinction rates are typically followed by a recovery with high species origination rates within a few million years. The availability of nutrients is a key factor in regeneration of life; hence, for animals, the criticality of algae and plants. Adaptive ability during an ongoing extinction event determines the event’s severity and longevity in relation to biota.

▫ The causes of extinction events are various, ranging from sudden impacts from space, radiation, geomagnetic reversals, volcanism, continental drift, and global climate change. These factors are often intertwined in increasing the severity of mass extinction.

▫ The worst extinction event to date was the Great Dying, at the Permian–Triassic (P–T) period boundary, beginning 252 MYA. The mass extinction event underway now may be its equal.


▫ Eukaryotes arose 1.5 BYA, after a 2-billion-year dominion by prokaryotic life. Multicellularity begat complexity as well as diversity. Sophistication is something else: evolution has only produced a difference in kind, not degree.

▫ The fossil record of oceanic animal forms exploded during the early Cambrian period, 540 MYA.

▫ The first fish appeared 530 MYA. Vertebrates – fish with backbones – emerged 500 MYA.

▫ Sharks evolved 420 MYA, during the Silurian, and became a most successful order, in both the oceans and fresh water. Shark success owes much to its elegant simplicity and its diverse array of senses.

▫ The earliest plants appeared on land 450 MYA, during the Ordovician. Animals came in pursuit. The first were arthropods the size of fleas. Insects evolved on land 400 MYA. In an escalating evolutionary war, plants evolved to cope with pesky herbivores.

4 major plant groups have successively dominated terrestrial flora. Each group retained traits that had proven successful. Differentiation came by innovation.

The earliest vascular plants originated in the Silurian, 430 MYA. Vascular plants were able to colonize the land by evolving superior water management. This involved both water transport from the ground and controlling water loss in the air.

The development of spores characterizes pteridophytes, which arose 390 MYA during the Devonian. Next came seeds: gymnosperms, in the early Carboniferous, 340 MYA.

Flowers were the latest innovation. Angiosperms first bloomed during the mid-Triassic, 245 MYA. Their diversification is still somewhat early in the speciation cycle.

▫ With each new reproductive grade, plant species rapidly radiated, plateaued, and witnessed a gradual decline in species number. Unlike animals, there has not been a great loss in plant diversity except at the Permian–Triassic boundary 252 MYA.

▫ The first vertebrates crawled from the water onto land before 360 MYA, when an extinction event wiped out half of the extant species. Recovery was underway within 10 million years. By 335 MYA, amphibious fish that had come on land evolved to amphibious tetrapods.

▫ Large dinosaurs evolved during the Triassic. Dinosaurs lived on Earth 180 million years, as a dominant group ~140 million years, to the end of the Cretaceous. The reign of the dinosaurs is dwarfed by sharks and cockroaches, who have lived on Earth twice as long as the dinosaurs ever did, largely unchanged for hundreds of millions of years.

▫ The compounded occurrences that did in large dinosaurs ended the Cretaceous period 66 MYA. The Cretaceous–Paleogene (K–Pg) extinction event began with continental drift altering worldwide climate. Then a massive Indian volcano shot deadly soot into the sky, further cooling the planet. Finally, a bodacious bolide struck Chicxulub in Mexico’s Yucatán peninsula, capping off the planetary biotic decimation. For all that, K–Pg was the least intense of the major mass extinction events.

▫ Only a single lineage of the smallest dinosaurs managed to survive the K–Pg mass extinction: birds.

▫ Long-term climate changes can be a product of Milankovitch cycles: fluctuations in Earth’s orbit, the tilt of Earth’s axis of rotation, and the proximity of Earth to the Sun, by time of year. Global redistribution of vegetation correlates with Milankovitch cycles.

Milankovitch cycles are not evolutionary impetuses to plants. Tectonic pulses provoke plant evolution.

▫ Modestly sized mammals evolved around the same time as dinosaurs. Mammals bid their time underfoot the enormous roaming reptilian descendants. The K–Pg mass extinction opened the door to mammal proliferation.

▫ Primates evolved 60 MYA. The earliest ones were mouse sized. Apes evolved from Old World monkeys 25 MYA. From apes came the first hominoid 23 MYA. Much speciation and descent of mixed lineages eventuated in humans roughly 300,000 years ago.