The Time Line
(updated: February 17, 2004)
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To understand origins and evolution of life, we should have a general understanding of time. The universe first started with The Big Bang. The estimates for when the Big Bang occurred, has ranged from 10 billion years ago (BYA or 10^9 years) to 20 BYA. Recent estimates have put the age of the universe to about 13 or 14 Billion years old.
Immediately after the Big Bang, the Universe was extremely hot and extremely dense. The Universe cooled down as it rapidly expanded, which eventually allowed for the stable formation of atoms (Hydrogen and Helium) and subsequently, the stars and galaxies (see Table-1). Earth's sun formed around 8 Billion Years Ago, and Earth itself about 4.6 BYA (see Table-2). The oldest microfossils in the geological record are about 3.5 BY old, and there is some indirect geochemical evidence that life may have arose on Earth 3.8 BYA (Mojzsis et al., 1996). Now, the Earth's crust couldn't form until the planet's surface cooled sufficiently, which was approximately 0.6 billion years after the Earth formed, so that leaves only about 200 to 500 million years for life to arise on Earth. That is a very short time, relatively speaking. The question as to how life first arose (e.g. what events occurred or what processes were involved) remains unanswered. However, there are a number of theories as to how life originated, and some of these will be outlined in a different section.
Once life arose on Earth, a number of major events occurred during biological evolution (for a great reference on the topic, see Maynard Smith and Szathmary). The first eukaryotes are thought to have appeared around 2 billion years after the first prokaryotes. The geological record indicates that land plants and land fungi existed about 480 to 460 million years ago, and there is some evidence that plants colonized the land around 600 to 700 million years ago (Heckman et al., 2001). Atmospheric oxygen levels rose to significant levels at this time, and have remained relatively constant since. Multicellular life is associated with this oxygen increase (probably due in part to the generation of the ozone layer which helped to block out harmful radiation). Throughout the history of life, there has been a number of evolutionary explosions that led to rapid increases in species diversity (e.g. the Cambrian explosion). New species evolved within relatively short time periods. There were also a number of catastrophic events that led to the decline in species numbers through mass extinction events. For example, dinosaurs became extinct towards the end of the Cretaceous period in the Mesozoic Era (Table-3), at around 65 million years ago, due to an asteroid impact.
In the current Epoch (the Holocene, of the Quarternary period in the Cenozoic) modern Homo sapiens evolved. To put things into perspective, for the enormous amount of time available during the evolution of life on Earth, take the estimated time modern humans have existed (i.e. about 10 000 years). The percentage out of 4.6 billion years that humans have existed is 0.0000021 percent. Thus, humans have only been around for a blink of an eye when compared to the long history of the Earth. NOTE: tables are adapted from various sources. Most of Table-3 is modified from Futuyma, 1998. References:
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