The Earth’s geological and biological history has been divided into a number of: eons, eras, periods, epochs and ages. These have been extensively studied by scientists and are defined by the Geological Column, also known as the Chronostratigraphic Chart (a link to a Google Docs spreadsheet).

One of the early Periods has been labelled the Cambrian Period, and it began about 550 million years ago and ended about 490 million years ago. The entire Cambrian Period therefore lasted for almost 60 million years. It should be noted that the Earth was around for about 4 billion years prior to the Cambrian period, and there is evidence of life from at least 3.5 billion years prior to the Cambrian.

The following image (taken from the Wikipedia article about the Cambrian explosion) shows the placement of the “explosion” in relation to the established development of life on Earth:

About 10 million years into the Cambrian, and lasting for up to 25 million years, an interesting thing happened: practically all of the major animal phyla (the classification below Kingdom and above Class) started appearing in the fossil record. This phenomenon has become known as the Cambrian explosion, and it’s no surprise that the event has been hijacked by some religiously-minded as “evidence” that their god must have kicked things along at about that time.

The Young-Earth creation extremists of course believe that all of the 100-plus layers of the geological column were laid down (in an orderly manner all over the world) in a huge flood about 4,500 years ago (no, seriously, they do), so it’s some of the Old-Earth creationists who have come to somehow believe that god-dunnit™ when it comes to the “explosion”. But think about that for a moment: they actually believe that their god created the universe via something resembling a “big bang” over 13 billion years ago, then waited about 9 billion years to create the Earth (with a little bit of very primitive life following soon after), and then waited close to 4 billion years to really stir things up by causing an “explosion” in the diversity of life on Earth.

Wrong. Why would a deity do that? Just to make it really look like natural forces were at work and that evolution is a real thing? Weird!

No matter what sort of “explosion” the evidence suggests, that in no way can be evidence for a deity. Which particular fossil finally convinced a creationist that a deity must have been responsible, and what’s more, that the deity must be of their particular denomination?

And consider the “explosion”, which actually took about 25 million years. Most people have trouble conceiving what 1,000 years really means, yet the “explosion” was a period 25,000 times longer again. It would have to be, by definition, the slowest explosion in history. There are an awful lot of generations in 25 million years for primitive organisms to evolve if the conditions are right.

And “conditions” is the key to the issue. We take the existence of oxygen for granted however it turns out that oxygen took an awfully long time to get going, and is actually the product of billions of years of photosynthesis. Initially the produced oxygen reacted with iron and other minerals in the rocks and oceans, and it was only after they were saturated that oxygen began to exist in the atmosphere. And while there is no convincing correlation between oxygen levels and evolution, it can’t be denied that life as we know it today is dependent on various required amounts of oxygen in their environment (which had developed by the Cambrian period).

Also important was the increase in the amount of ozone – which is believed to have existed around the Cambrian period. Ozone is required to shield the Earth from biologically-lethal UV radiation, and was necessary if complex life was going to develop on the land as well as in the oceans.

Research has also revealed that volcanic activity at the time caused a massive and sudden surge of the calcium concentration in the oceans, thus making it possible for marine organisms to build skeletons and harder body parts.

It is also likely that once the diversity of life started, an arms race between predator and prey contributed to the “explosion” that was to follow.

The evolution of the eye (and its primitive precursors) is believed to have been in full swing about 555 million years ago, and that is just prior to the start of the Cambrian period. One theory is that the predator-prey relationships changed dramatically after eyesight evolved. Due to increased vision, different hunting techniques could happen and therefore better defenses (e.g. with armor and spines) were needed for survival.

It must also be noted that prior to the Cambrian, the fossil record becomes noticeably poorer, however that is attributed to the much higher sedimentary deposits during the Cambrian and also because the animals prior to the Cambrian were much “softer”, e.g. lacking resistant shells (meaning that their survival as fossils was less certain). So while there may have been a tremendous amount of life prior to the Cambrian, it was less diverse, more primitive, and didn’t leave a good fossil record.

It is known for certain that life existed prior to the Cambrian period, so it’s not the case that the “explosion” created life.

There are many other theories for the diversity of life that occurred during the Cambrian period, but no scientist believes that something supernatural is needed to explain what the evidence has revealed.

Why do you?