Life on Earth: the highlights – Part 2

In part 1 I looked at how the Earth formed, how quickly life emerged and the key events in the history of the Earth that shaped the evolution of life, like the Earth becoming a giant snowball.

In this post I’ll carry on where I left off, from the non-explosion that was the Cambrian explosion, to the emergence of the dinosaurs.

As with the previous post I’ll give approximate times for each event millions of years and I’ll use the label Ma for millions (mega-annum). I’ll also show the time in terms of a 24-hour clock, as it’s easier to understand and more fun, with time on Earth beginning at 00:00 and 23:59 being today. The last event I looked at previously was the evolution of the first shells, legs and eyes, at 542 million years ago, or 21:01 on the 24-hour clock. The next event is…

The first fish swims in the sea; (530 Ma) 21:12 – fish were the first true vertebrates (animals with backbones) to evolve. All vertebrates alive today, including humans, evolved from fish, so they’re a pretty important evolutionary step. At first they were very simple animals with no jaws, but over time they radiated throughout the oceans becoming increasingly complex.

The first plants invade the land; (450 Ma) 21:38 – although microorganisms and simple life, like algae, and even animals like insects had already moved onto the land much earlier, the first true plants emerging onto dry land was a hugely important step, as photosynthesising plants form the basis of all land ecosystems. Early plants include the paleobotanists favourite Cooksonia. Once land plants were established, life could really begin to conquer the continents…

The first tetrapods waddle onto the land; (395 Ma) 21:55tetrapods are animals with four legs; they include amphibians, reptiles, birds and mammals (that includes us). The oldest fossil evidence of tetrapods on land is from fossilized footprints of an unknown animal. Early tetrapods, like Tiktaalik, Acanthostega and Ichthyostega, lived in the water, they breathed air but likely used their legs for swimming and digging, rather than walking on land. But soon tetrapods, like Casineria, used their legs to their advantage and moved onto dry ground. Tetrapods moved into ecological niches not yet occupied by other species, encouraging their evolution and diversification…

And God created amphibians; (360 Ma) 22:06 – only joking, it wasn’t really God, it was Charles Darwin. Amphibians represented another important evolutionary innovation for terrestrial life, as tetrapods were becoming increasingly adapted to life on land. Amphibians begin life as a larval form, like tadpoles, which are better suited to living in water, usually having gills. But the larva usually metamorphose into an adult form, with air-breathing lungs, and bodies suited to travelling over land and eating invertebrate prey like insects and worms. It’s strange to think that for millions of years amphibians were the apex predators on Earth.

The first reptiles evolve from amphibians; (300 Ma) 22:25 – reptiles were another key step in the evolution of tetrapods. Whereas amphibians can live on the land, they need to stay moist to stop their skin drying out, and they need to lay their eggs in water. Reptiles found solutions to both of these problems, their scaly skin stopped their bodies loosing water, and they laid amniotic eggs, with leathery shells which protected them from the harsh environment on land and allowed air to diffuse in and waste to diffuse out of the egg. Reptiles quickly became the dominant form of life on Earth, spawning such awesome animals as dinosaurs and crocodiles. Nuff respect.

Life on Earth almost dies; (251 Ma) 22:41 – an enormous extinction-event occurs, called the end Permian mass extinction. Potentially as much as 90 to 95% of species on Earth are wiped out. The cause of the mass extinction is not clear, but a period of immense volcanism, called a flood basalt, did occur, and this may have initiated dramatic climate change, potentially by releasing methane trapped in the ocean into the atmosphere causing rapid global warming.

The dinosaurs’ stomp and roar onto the scene; (225 Ma) 22:49 – actually they begin as small two-legged reptiles, like Eoraptor, but after the late Triassic extinction event dinosaurs diversify into a huge range of species; from the hulking herbivorous Sauropods, to giants like Spinosarus to nimble predators like Deinonychus. Dinosaurs are sometimes portrayed as evolutionary failures, but they successfully ruled the Earth for another 160 million years! Also, some dinosaurs evolved into birds, so the ancestors of the dinosaurs are still with us today. They were an immensely successful group of animals.

Next up in part 3, the evolution of mammals to the emergence of apes that drive cars (that’s us).

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2 Responses to “Life on Earth: the highlights – Part 2”


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  1. 1 Life on Earth: the highlights – Part 1 « Astrobioloblog Trackback on January 28, 2011 at 12:16 pm

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