Scientists have predicted that the rise in oxygen levels in the atmosphere prompted aquatic life to move outside of the Ocean, where organisms were constantly fighting for Oxygen. A paper published by Dahl, et. Al. on September 28, 2010 (yesterday) provided the first evidence of this. Unfortunately, I could not find this paper yet, but I found a summary at Wired Science. The people involved in this study tested prehistoric seafloor samples (dating from 1.7 billion years ago to 400 million years ago) found all around the world. They were testing for molybdenum, a mineral found in soil and carried by erosion. In the Ocean, it takes particles about one million years to stop circulating and finally rest on the ocean floor. The lighter isotopes of molybdenum, in oxygenated water, sinks into the seabed. This means that deposits left behind are a stratified record of Earth's oxygen composition. The levels of oxygen in the water are believed to also reflect the levels of oxygen in the atmosphere.The main author of the paper, Tais Dahl, says that oxygen is a more detailed record that what can be read in carbon. Because of indeterminate data in carbon dating, there are two competing theories about Earth's early oxygen levels. Each theory accepts that oxygen levels first spiked about 550 million years ago along with the first mobile, symmetrical life forms. The traditional model states that oxygen levels steadily continued to rise, reaching levels like our modern levels, before organisms diversified again (approximately 400 million years ago). This model supports that it only took about another 50 million years before aquatic organisms came to land. The second model hold that oxygen levels stayed at a steady rate between 550 million years to 500 million years ago. This is when prehistoric plants evolved and diversified. This is when oxygen levels spiked, allowing fish to evolve into more predatory forms. This second model is supported by tests of molybdenum. Plants (release oxygen both while they live and decompose) are the key difference in the two hypotheses. According to Dahl, “The low oxygen level early in animal history limited evolution for fish. After this second oxygenation event, we begin to see large, predatory fish up to 30 feet long. When land animals walked out of water in the first place, it was to escape predation. It’s oxygen that drove the evolution of large predators in the ocean. It’s plants that caused oxygen to rise. In principle, you could connect this all.”
Here is the citing from the recently published paper.
“Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish.” By Tais W. Dahl, Emma U. Hammarlund, Ariel D. Anbare, David P. G. Bond, Benjamin C. Gill, Gwyneth W. Gordon, Andrew H. Knoll, Arne T. Nielsen, Niels H. Schovsbo, and Donald E. Canfield. Proceedings of the National Academy of Sciences, Vol. 107 No. 39, September 28, 2010.
Summary of paper found at Wired Science by Brandon Keim.
To learn more read Nature News
Cartoon from http://fishfeet2007.blogspot.com/2007/04/fish-find-land-legs.html
Image from http://www.dailymail.co.uk/sciencetech/article-1084251/Scientists-discover-chance-meeting-1-9bn-years-ago-led-life-Earth.html