Changes in the Earth: On the way to life (and beyond)

EPS 103

Lecture 11, Oct. 3^rd, 1999

The Earth is defined as being constantly changing, yet overall staying the same. Nevertheless, there have been a number of crucial geological and biological developments that changed the face of the planet forever. These are outlined below. (A whole course could be devoted to this subject, not just one lecture).

Hadean Eon: 4.6 to 3.8 billion years: The Earth’s violent beginnings – before life.

The Earth is 4.65 billion years old. And yet, there are no rocks on Earth much older than about 3.8 billion years old. (There are tough mineral inclusions in younger rocks that have a 4.2 billion year old radiogenic date). Why nothing older? Because the Earth was bombarded with planetessimals in the early stages, and the surface was completely reworked and even molten. No remnants remain. How do we know it is so old? Moon and meteorites. They are the early clue to our own planet.

During early formation, the constant bombardment of meteorites and planetesimals probably blew off any atmosphere that was there (hydrogen, helium, argon, nitrogen, neon). Then, as things cooled off a little and the intensity of meteorite bombardment subsided, an atmosphere slowly built up from degassing of nitrogen, sulfur, CO₂ and H₂O gases via volcanoes and hot springs. (This is still happening today in lesser amounts).

The Hadean Earth was a hostile place for life. Hot and oxygen free (no diatomic oxygen or O₂). The atmosphere was dense, with lots of water vapor, CO₂, sulfur and nitrogen gases. When these reacted with water they produced HCl (hydrochloric acid), H₂SO₄ (sulfuric acid), HNO₃ (nitric acid), etc. Not a nice place for life!

Precambrian Eon (3.8 to 0.545 Ga[1]): Radical Atmospheric changes and slow beginning of life

Archean Eon (3.8 to 2.5 billion years ago). By this time, the hostile acidic atmosphere is gone, but still no free oxygen. It is warm, so there’s lots of water in the atmosphere as well as CO₂. (Perhaps 100 times more CO₂ than today). The sun’s luminosity was perhaps 25% less than today. This leads to a paradox. If the sun was less intense, why didn’t the Earth freeze over? It seems that it was at least as warm as today because of the greenhouse effect. The excess CO₂ and H₂O trapped the heat from the sun from escaping out of the atmosphere. The temperatures may have been as high as 60°C.

How did the CO₂ get removed from the atmosphere? It reacted with calcium silicate rocks, dissolving them. Then calcium carbonate (CaCO₃) was reprecipitated in the Ocean. Once life evolved, carbon could also be removed from the atmosphere as organic matter. (Organic matter is mostly hydrocarbons, which are chains of hydrogen and carbon atoms). By removing CO₂, the planet would cool, more water would be removed from the atmosphere, cooling it further, and productivity and erosion rates would slow (negative feedback), so that the rate of CO₂ loss would lessen. How does the CO₂ get recycled to the atmosphere? Via plate tectonics! When plate tectonic activity is high, then there is lots of recycling, the quantity of CO₂ in the atmosphere is high, and so are temperatures. Low plate tectonic activity, less CO₂ in atmosphere, lower temperatures. In fact, if CO₂ were not returned to the atmosphere via plate tectonics, the planet would cool down and become frozen, much like Mars is today. So, without plate tectonics, life itself would not be possible!

Life begins

With time, the rocks reacted with the acids, neutralizing them. Now the planet is ready for life. Where, how and when it began is not known, but fossils go back as far as 3.5 billion years.

Several suggestions exist for the origin of life:

· Small warm ponds in the ocean (Darwin’s idea)

· Dried-out ponds in the ocean! (able to polymerize amino acids into long chains).

· Deep mid-ocean ridges (protected from harmful UV radiation, lots of ‘food’.

· Extraterrestrial (life on Mars? There are definitely extraterrestrial amino acids on
Earth.

The earliest life forms were prokaryotes (before nuclei). They appear to have been photosynthetic. These are primitive bacteria. Blue-green algae are found in fossilized sedimentary rocks from Australia.

The prokaryotes cannot tolerate free oxygen. Although they consume CO₂ and give off O₂ as a byproduct, they cannot tolerate any large quantities of free oxygen. Their release of oxygen into the atmosphere led to the protective ozone layer being developed. This allowed organisms to come out from behind rocks and into the sunlight! They kept to the fetid, oxygen-poor muds of the ocean.

Proterozoic (2.5 to 0.545 Ga)

Iron formations

Between 2.2 and 1.6 Ga, voluminous deposits of iron formations occurred. These are iron rich sediments. Their presence indicates that the atmosphere was still O₂-poor (anoxic). Iron is soluble in oxygen-free conditions (as Fe²⁺), but not in oxygen-rich conditions (as today). With free O₂, iron forms insoluble iron oxides. The iron formations were the culmination of oxidation of reduced, dissolved iron in the oceans. As long as there was sufficient iron to keep the O₂ content low, no free oxygen could occur.

Eukaryotes – Around 2 Ga, eukaryotes developed. They had membrane-bound nuclei and could thrive in oxygen-rich environments. Originally, they were single-celled algae, later multicelled algae (seaweed). With time, the eukaryotes split into two evolutionary trends – the plants, and the animals. Around 1.6 Ga, the oxygen content of the atmosphere was building up. It would take another billion years to reach modern levels.

The Precambrian era came to a close about 545 M. years ago. It represents a whopping seven-eights of the Earth’s entire history, and very little is known about it.

Phanerozoic Eon (visible life).

The Phanerozoic eon is divided into the Paleozoic (545-245 Ma), the Mesozoic (245-65 Ma) and the Cenozoic (65 – 0 Ma).

The Phanerozoic is divided into different Eras (Paleozoic, Mesozoic and Cenozoic). These are further divided into a whole host of periods. These are generally classified on the basis of extinctions.

The overall picture is given in the following diagram: There are obviously lots more details.

Paleozoic – the rapid blooming of life

The Paleozoic is divided into 6 periods: The Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian periods.

Cambrian – The atmosphere was similar to what it is today. This was a time of explosion of life. There was an explosion of marine life. Hard-shelled organisms developed, so that we have an extensive fossil record of evolution during this period. Trilobites, sponges, snails, clams and corals. The explosion may be related to the development of sex. Prokaryotes are asexual. The development of sexual reproduction would lead to a rapidly evolving community.

(Previous page – a thriving Cambrian community from the Burgess shale (British Columbia)

Ordovician period – Fish begin to populate the seas.

Coming to land. In the Silurian period, plants began to occupy the land. Three requirements are needed for this development.

1. Self support. The water no longer supports the structure
2. Internal aquatic environment. Controlled fluids.
3. Means for exchanging gases with atmosphere instead of water.

Creepy scorpions crawled out of the sea! This is the golden age of fishes.

Devonian – Fish crawled out of the Ocean onto land in the Devonian period, quickly giving rise to the amphibians. Insects joined them. This is the time of spiders, scorpions, cockroaches, etc. The Devonian period is also the time of great fish in the sea. Seeded plants

Carboniferous -- This was a hot period when the Earth was covered with vast forests and swamps. Tree-sized conifers and huge ferns covered the land. Much of the coal on Earth is from this period. The removal of so much carbon from the atmosphere led to high oxygen levels, very nice for plants. Widespread forests of giant club moss trees, horsetail and tree ferns.

Early reptiles, first winged insects. Amphibians rule!

Permian – Plate tectonic activity slowed, as the super continent Pangea developed. The lack of mid-ocean ridge activity led to a cooling, and eventually an icehouse-like condition, with evidence of continental-scale glaciation. Driven by plate tectonics!! The Permian-Triassic boundary is the most devastating example of extinctions in the Earth’s history.

Mesozoic Era – 245-65 Ma (middle life)

The Permian extinction opened new ‘niches’ for species to fill. By late Triassic, Pangea was breaking up, plate tectonic activity was on the rise. Australia and Antarctica separated to form a continent 150 Ma, then divided 50 Ma. For much of the Mesozoic, the Earth was a warm place again, conducive to life. Reptiles and primitive mammals are populating the planet.

The Triassic ended with a severe extinction (200 Ma). The Jurassic opened the way for the dinosaurs, which continued into the Cretaceous. The Cretaceous saw the beginning of flowering plants. Explosive radiation of dinosaurs. First primative mammals. Beetles begin (Now number over 1.5 million species!!!).

Jurassic This was a time of a Steven Speilburg movie. This is the great age of dinosaurs (controlled both houses of Congress and the Presidency). First bird – archaeopteryx. Forests of gymnosperms and ferns.

Cretaceous First flowering plants (angiosperms). Modern birds, dinosaurs go extinct after 150 million years – not too bad a showing!

It all came to an end with the meteorite impact at 65 Ma. The Cenozoic begins.

Cenozoic Era – 65 Ma to present.

The Cenozoic includes the Tertiary Period (65 – 1.6 Ma) and the Quaternary Period (1.6 to present).

Tertiary includes: Paleocene, Oligocene, Miocene, and Pliocene epochs.

Quaternary includes: Pleistocene Epoch (1.6 Ma to 10,000 years ago) and the Holocene epoch (10,000 – 0 years ago).

Tertiary Explosive radiation of angiosperms (flowering plants) and massive radiation of mammals. By the mid Tertiary, all modern genra of mammals present. Towards the end of the Tertiary (~3 Ma) early hominids appeared.

With the demise of the dinosaurs, the little, non-specialized mammals took over. They filled the niches left by the dinosaurs, evolving and specializing. Carnivores, ungulates (hooved animals), etc. took over. There was a great diversification, with large carnivores developing. The Cenozoic is the Age of Mammals. Lots of ocean shelf organic deposits formed, creating half of our known oil reserves.

Tectonically, India broke away from Antarctica and crashed into Asia 50 Ma. Australia separated from Antarctica 50 Ma, developing a separate fauna.

Around 35 Ma, a 4^th icehouse condition began, that culminated in the Pleistocene epoch. The Pleistocene includes a period of warmer and colder periods called glacial and interglacial periods. There have been 10 major glacial periods. We are presently at an interglacial period.

The end of the Pleistocene (10,000) resulted in a major extinction of the large mammals. Why? Was it the effect of man? No one is sure.

Today’s extinction rate? Perhaps ¼ of all species in the next 30 years.

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[1] Ga refers to ‘gigayears’ which is equivalent to billion years. 3.8 Ga is 3.8 billion years.