The planets formed from the solar nebulae. The mass M of the planets roughly relates to the distance to the sun R according to M ~ R². Jupiter is too big and stole mass from the asteroid belt, which then became too small to make its own planet.

The earth consists of "processed material" and formed a core-mantle-crust ‘onion’. The original material or primitive solar matter is only found in rare meteorites (carbonaceous chondrites).

The original accretion of the earth was cold, but the earth heated up as a result of conversion of kinetic and gravitational energy into thermal energy and from the heat generation of radioactive elements. The earth started to melt and formed a magma ocean. The blebs of Fe that we find in many meteorites started to melt first and slowly settled to the center of the earth (density separation). This led to more heat release and many gases were expelled in this event (Big Burp).

The atmosphere of the earth is a secondary atmosphere that formed during the Big Burp. The Big Burp gases (CO₂, CH₄, N₂, H₂O) were volcanic gases, whereas the primordial atmosphere (H, He) got blown away during early convulsions of the sun (T-Tauri winds).

The early sun was much "cooler" than the modern one (about 75 % of modern solar energy output). Why did the earth not freeze over? There is evidence for running water in the structure of rocks (ripples, layering) that are up to 3.8 Ga. The CO₂-CH₄ rich early atmosphere created a very effective greenhouse: the small amount of sunlight that reached the earth was effectively trapped.

Variations in concentrations of the greenhouse gases led to several "snowball earth" events (e.g., around 1.7 Ga). Rock evidence suggests that former tropical areas (reef like environments a la Bahamas) were directly overlain with glacial deposits (like moraines). Presumably, the whole earth was covered by ice except small havens around volcanoes, so life may have survived in these volcanic hot springs. How did we get rid of the ice?

The albedo of ice (reflection of sunlight) is a strong climate feedback during snowball earth, and would not allow a return to a milder climate. However, emission of volcanic CO₂ for a long time would lead to a supergreenhouse and rapid warming, with melting of the icecap. The CO₂ could not go anywhere because no dissolution in water occurred given all the ice. After ice melting, the albedo also disappeared and it became really hot (but no boiling-off of the oceans probably occurred). So we saw an alternation of snowball to hothouse conditions which may have tested the resilience of life. After this period of rapid climate upheavals, we find the Cambrian explosion, a period of rapid divergence in life forms at about 0.5 Ga. Presumably, many ecological niches had been emptied during the harsh climatic conditions, and when the climate stabilized, these open niches were occupied by opportunistic species. These then developed and adapted to their environment, and life progressed in all its morphological and functional glory.

Evidence of early life in the form of recognizable shapes occurs already at 3.5 Ga. However, there is only evidence for the build-up of atmospheric O₂ around 2.5 Ga. If the early life was based on photosynthesis, where did all that O₂ go? The photosynthetic reaction at its simplest can be written as CO₂ + H₂O + light -> CH₂O + O₂. For each molecule of organic matter formed, one molecule of O₂ is released to the atmosphere. We thus expect that all buried C_org from all geological time would be equal to the amount of O₂ in the atmosphere (if we assume that all free O₂ in the atmosphere comes from photosynthetic processes only). Buried C_org is largely found as finely dispersed dark specks in rocks. When we add that all up, we find a lot more carbon in old rocks than O₂ in the air. Where did all that earlier produced O₂ go? (This is in agreement with the lack of O₂ in the early (2.5 — 3.5 Ga) atmosphere so  some O₂ must have ‘disappeared’).

Many elements in the solid earth occur in low valency states (e.g., Fe²⁺, H₂S). These compounds can be oxidized resp. to Fe³⁺ (rust) and sulfate (SO₄⁼) under the consumption of O₂. It is generally recognized that in the early earth almost all the produced O₂ disappeared into the earth through such oxidation processes. Only when the earth became overall more oxidized did this process slow down, and the amount of O₂ produced by "plants" started to exceed this lithospheric oxygen sink. By about 2.5 Ga, the rate of O₂ production was higher than the rate of O₂ consumption and free O₂ started to appear in the atmosphere. Clearly, the early forms of life must have been able to live in close to "anaerobic" circumstances.

So the history of the earth is characterized by violent climatic fluctuations and chemical changes in the composition of the atmosphere. Life has survived it all, but of course not all species did. Once we start comparing the person-made environmental changes with these older fluctuations, the true magnitude of our pertubations become obvious.