Evolution and Extinction
On geological time scales, time is organized into eons, eras, and periods. There are four main eons subdivided into different eras. The eras are in turn split into different periods. The current eon, the Phanerozoic Eon (574 Mya through today), is known as "The Age of Life." The Cambrian Period is the earliest period of the Phanerozoic eon, lasting from 540 to 500 Mya. The events leading up to this period set the stage for an explosion of complex life. Changing atmospheric oxygen concentration allowed for more efficient aerobic metabolism and increasingly complex life forms.
The Proterozoic Eon began 2.5 billion years ago and set the stage for Phanerozoic life. From the geologic record, we know that oxygen levels were low before the Proterozoic. Therefore, microbes from the start of this era would have primarily been anaerobic chemotrophs, producing energy through chemical pathways.
The Proterozoic Eon was likely a time of rapid diversification of life. Today, there are systems of proteins that run checks during the replication of DNA to minimize transcription errors. However, these types of proteins are less common in prokaryotic microbes, which were common during the Proterozoic Era. A higher rate of mutations would have accelerated natural selection and a driven greater diversification of organisms. One of those mutations enabled photosynthesis, a more efficient process of respiration, resulting in an explosion of complex multi-cellular organisms that appeared at the start of the geologic era known as the Cambrian Explosion.
The Cambrian Explosion
At the time of the Cambrian explosion, there was an exponential increase in the number and complexity of organisms. Complex organisms likely preferred living in the oceans at first, where there was easy mobility and easy access to nutrients. Even very shallow levels of water provide protection from damaging UV radiation.
The earliest evidence of complex, land-based life comes from early land plants that begin to appear in the fossil record around 475 Mya. DNA evidence suggests that these plants evolved from algae, a general term for aquatic, photosynthetic organisms. It is a likely transition given that many breeds of algae prefer growing in shallow water to begin with. Algae adapted protections, such as thicker cell wells, in order to survive dry spells that would have been beneficial in the transition from water to land.
With other organisms still constrained to the water, land plants flourished and grew large in size. The lack of competition further allowed for increasing complexity: the development of root systems and tubes to transporting water and nutrients throughout the plant. Photosynthesizing land-based plants had abundant energy from the Sun.
The Carboniferous Period
Forests began to cover the lands and dead organic matter began to pile up and decompose among the plants. Millions of years later, this organic matter would be compressed into coal, earning this period the name "carboniferous," which means "coal carrying'' in Latin.
The Carboniferous period began roughly 360 Mya and lasted about 60 million years. It was marked by an era of giantism. The fossil record from early in this period is rich in ocean and fresh water organisms. Land-based organisms appeared in the middle to later part of the period. The first terrestrial organisms were supersized insects and amphibians. By the middle of the Carboniferous period, adult amphibians were up to 6 meters long and developed the scaly skin typical of lizards by the end of the period. The Arthropleura, which resembles a 2.6-meter long millipede, proliferated during the late Carboniferous period and remains the largest-known land invertebrate ever discovered.
The Carboniferous period also gave rise to the largest-known flying insect: the Bolsover dragonfly. The Bolsover dragonfly looks nearly identical to the dragonflies common today, but with a wingspan of 30 cm - six times larger than a modern dragonfly. With a wingspan this large, the Bolsover dragonfly would not be able to fly in today's atmosphere, with 21% oxygen. This has been used to suggest that atmospheric oxygen levels may have been as high as 30% during the Carboniferous period. Increased levels of oxygen as well as a generally moister environment would explain the large size of organisms at the time. Respiration was likely more efficient so that organisms could produce more energy and sustain their larger sizes.
As life continued to evolve and diversify, ecosystems began to resemble the types of settings we are familiar with today. This penchant for giantism continued into the Mesozoic era, which spanned from 252 to 66 Mya and is known as "the Age of the Dinosaurs.'' The Cenozoic era that followed saw the emergence of mammals. The end of each period during the Phanerozoic Eon was marked by mass extinction events that completely upended the biological landscape of the time.