For background in this section you will need to read two articles:
- Lunine, J. I., 2006. Physical conditions on the early Earth. Philosophical Transactions of the Royal Society, B, v. 361, p. 1721-1731.(link is external)
- Knoll, A. and J. Grotzinger, 2006. Water on Mars and the prospect of Martian life. Elements, v.2, p. 169-173.(link is external)
So, the "Goldilock's Principle" postulates that everything was "just right" on Earth for life to originate and prosper. Did water play a role? Make a list of all the ways that water could be important to the evolution and continuity of life on Earth (think broadly). For example, if Venus and Mars once had water, and even oceans, why do they not now have them? Clearly, if life arose in the presence of water, that water would have to persist in order to sustain life. Could life have evolved on Mars? Where would you look for life on Mars today?
One of the key constraints on the accumulation of oceans at the Earth's surface and the origin and survival of the earliest life on Earth is the size and frequency of objects that impacted the Earth. Lunine (2006) summarizes the impact history on Earth (largely inferred from the preserved record of impacts on Earth's Moon; why not directly from the earthly record?). Note that in the first 0.3 billion years (4.5-4.2 Ga) after Earth's accretion, the frequency and size of impactors was such that multiple "sterilizing" impacts occurred. In addition, these impacts probably "blew away" any oceans that may have accreted early and created a "steam" atmosphere. Certainly, some water was lost from Earth's surface to space. Fortunately, sufficient water existed either through accretion or continued addition by comets and asteroids (section 1) that oceans could again accumulate on Earth's surface. But life could have originated multiple times and been erased from Earth's surface by these large impacts. However, some models suggest that some life might have survived if it had evolved in higher-temperature environments, such as hot springs systems. In contrast, Venus and Mars somehow lost much of their water (and/or were initially endowed with much less than Earth?) during their early history, leaving Earth in the Goldilocks zone, and, perhaps, prohibiting an origination of life and/or continuity of life at their surface.
There is some evidence (what is it? See Lunine, 2006) for free water near Earth's surface as early as 4.4 Ga (the earliest known rocks extant on Earth) and fairly definitive evidence in rocks for large bodies of water (oceans?) by 3.6 Ga. Life may have arisen at that time, and there is reasonably strong evidence from structures and cellular features preserved in rocks that there were widespread mats of bacteria in shallow marine environments by about 3.3 Ga (Lesson 3 will entertain some hypotheses regarding the chemical composition of seawater). However, it took until nearly 0.54 Ga for multicellular marine animals to evolve. There is much speculation regarding the origin of life and why evolution took "so long" to allow more complex animals to exist. Little or no oxygen in the early atmosphere and oceans may have been a limiting factor, but there is disagreement regarding when the atmosphere-ocean system became "oxygenated." Available data indicate that some oxygen may have persisted in the atmosphere after 2.4 Ga, but more limited data may support an earlier timing for the "rise of oxygen." Note that oxygen can be considered a toxin to organisms that evolved in oxygen-deficient environments. Microbial organisms that once lived at the ocean surface would have been forced to seek refuge in oxygen-depleted environments below the seafloor when the oceans became oxygenated. Much work on this topic is going on in the Penn State Astrobiology Research Center as you read this.