The largest unit of defined geologic time is the supereon. Only one is defined, the Precambrian spanning from the formation of the Earth to right before life goes crazy in the Cambrian explosion (4.6 billion years ago to 542 million years ago). Oddly, there is no other supereon after the Precambrian, just the Phanerozoic eon ranging from the Cambrian explosion to the present. The Precambrian can be broken into three eons, Hadean, Acrhaean, and Proterozoic. Life as cyanobacteria first appears in the fossil record 3.5 billion years ago during the Acrhaean. Molecular estimates place life starting around 3.97 billion years ago. Did you get all that?
It doesn’t matter because this is all woefully inadequate. One of the most important events in the Earth’s history, the first appearance of life, does not mark the boundary of major time period. What do we call the entire timespan that life has occurred on Earth? This became issue when colleagues and I published a paper in 2009 describing the evolution of body size over the entire time span of life on earth (pdf). It has the rather gangly title
Two-phase increase in the maximum size of life over 3.5 billion years reflects biological innovation and environmental opportunity
Of course that is better than
Two-phase increase in the maximum size of life over Phanerzoic plus part but not all of the Precambrian reflects biological innovation and environmental opportunity
Or we could have said
Two-phase increase in the maximum size of life over Phanerzoic plus the Archaean and Proterozoic reflects biological innovation and environmental opportunity
Better would be a specific geologic timespan that we can invoke to refer to the time of life on Earth. My coauthors and I think we can fix this.
Behold the Supereon Geozoic (Geo=Earth and zoic=life)! The timespan on Earth sans life? Pregeozoic. The lower boundary would be set by the first appearance of life on Earth, somewhere between 3.8-3.2 billion years ago. The upper boundary cannot be defined until life has gone extinct on our planet. As stated in the paper, “The end point of the Geozoic can be only younger than today: life is still unequivocally present on Earth.”
Of course we also thought of other terms but (quote directly from paper)…”Admittedly, the terms Zoic and Azoic sound like names of evil characters from a low budget sci-fi movie and may thus be not mundane enough to be acceptable as chronostratigraphic terms.” Of course Zoic and Azoic also make no reference to geology and the Earth which we thought vital.
So why do really need the Geozoic?
- Conciseness. Take that title from above. It would have been so much better if it had been Two-phase increase in maximum size over the Geozoic reflects biological innovation and environmental opportunity. Similarly, ‘‘…little evolution at the macroscopic level took place for over half of the entire history of life on Earth’’ (Schulze-Makuch and Irwin, 2004, p. 39) could be ‘‘…little evolution at the macroscopic level took place for over half of the Geozoic’’
- Inadequacy of current terms. For example, how would you currently search for papers and research that examined the entire history of life on earth? Sure would be hand to have a single search term!
- Importance of the life’s origins. Use of the term would place a greater emphasis on the origin of life as a critical temporal event.
- Better organization of education materials. The Geozoic-Pregeozoic terminology would allow for better organization of introductory geological textbooks. From the paper, “In particular, the text dedicated to the origin of life and earliest life is often buried in chapters that deal with the whole Archean or even Archean and Hadean. This problem relates to the lack of explicit separation of the prebiotic and biotic Earth. The Geozoic offers a convenient tool for presenting the Precambrian history of life in a more structured manner in textbooks and in classrooms.”
- For comparisons on life on different planets. If or when we discover life on other plants, we might examine the different evolutionary trajectories on different planets. From the paper, “When we do so, we will need a convenient way to refer to the relevant time period on Earth as well as corresponding terms for other worlds.”
- Most importantly from the paper, “absurdly, we lack a name for the time of life on Earth”
To conclude, we encourage scientists to start using the term Geozoic in scientific publications and for the public to start using it at cocktail parties and water cooler chats
KOWALEWSKI, M., PAYNE, J., SMITH, F., WANG, S., MCSHEA, D., XIAO, S., NOVACK-GOTTSHALL, P., MCCLAIN, C., KRAUSE, R., BOYER, A., FINNEGAN, S., LYONS, S., STEMPIEN, J., ALROY, J., & SPAETH, P. (2011). THE GEOZOIC SUPEREON PALAIOS, 26 (5), 251-255 DOI: 10.2110/palo.2011.S03