Precambrian Period

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The Precambrian period is that long stretch of geological time (the longest actually) that starts from the formation of the Earth up to the beginning of the Cambrian – about 4.6 billion years ago. Just to give you an idea how old this is, dinosaurs did not come into existence until the late Triassic (230 Ma). They lived up to the end of the Cretaceous period (65 Ma).

Ma stands for Megannum, where mega means million and annum means year. Hence Ma is equivalent to millions of years ago wherein, in some contexts, the inclusion of ‘ago’ is already implied. Some texts use the abbreviation Mya instead; meaning Millions of Years Ago. In a few moments, you’ll be reading the term Ga, which means Gigannum (billions of years ago).

As mentioned earlier, the Precambrian period is not only the oldest geological period, it is also the longest. Since the Cambrian began some 540 Ma, then the Precambrian spans a little over 4,000,000,000 years. That’s almost 90% of the entire age of our planet.

The Precambrian period is divided into three eons: the Hadean (4.6 Ga), Archean (3.8 Ga), and the Proterozoic (2.5 Ga). You’ll sometimes read the term Cryptozoic in lieu of Precambrian. Crypt means hidden, and since a lot of the Precambrian is really hidden from our knowledge, some people prefer using Cryptozoic.

The reason why we know so little about this time is because most rocks have been buried deep beneath younger ones. Also, only a few fossils coming from this period have been found. In fact, majority of what we know about the Precambrian was only slowly uncovered in the past five decades.

Some well-preserved bacteria in Australia have been found to be at least 3.4 billion years old. Also discovered in the same area were fossils believed to be about 3.5 billion years old. This means that there must have been life in the Precambrian period, very basic though they may have been.

More evidence of life has been found at around 544 to 600 Ma. Although towards the end, that’s still definitely within the realm of the Precambrian.

Also found in Australia are zircons dated at 4.4 Ga. Since the said zircons exhibit oxygen isotopic compositions, it is believed that the Earth’s surface already had water as early as the Precambrian period.

There are only a few articles about the Precambrian period but we’ve got a few here in Universe Today:
Granite Rock
Ozone Layer

You may find more information about the Precambrian from the Michigan State University Department of Geography. There’s also some information about Precambrian rocks at USGS.

Check out Astronomy Cast. Episode 151 Atmosphere.

Tagged as: Cambrian, Precambrian Period, prehistoric