A group of Australian scientists have recently proposed that a severe depletion of selenium levels in the world’s oceans might have been a significant factor in three of the five mass extinction events over the last 550 million years.1 This, along with other data showing that both minimal and maximal amounts of selenium are essential to sustain animal life, suggests that selenium levels must be fine-tuned.
Selenium is a trace element essential for all animal life. The Office of Dietary Supplements of the National Institutes of Health (NIH) reports that selenium “is a constituent of more than two dozen selenoproteins that play critical roles in reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage and infection.”2The human body contains about 15 milligrams of selenium. Since too little selenium can lead to disease and even death, the NIH recommends a daily allowance of 0.055 mg of selenium from foods such as bread, grain, fish, meat, and eggs.
Yet too much selenium can also be harmful. According to News Medical, a British news organization, as little as 5 mg of selenium a day can be toxic for humans.3 (Selenium poisoning is generally not a concern since a person would have to consume selenium-containing foods in extreme excess to reach toxic levels.)
Current levels of selenium in the oceans range from about 60 to 200 parts per trillion (ppt) depending on geographic location and depth. The Australian scientists have projected that selenium levels dropped to about 1 ppt around the time of the three mass extinction events mentioned above. Of course, these scientists did not have access to ancient seawater samples. Instead, they determined the selenium concentration of 182 black shale samples that were age-dated and identified as having originated in ancient oceans over the last 550 million years. The researchers established a numerical relationship between the selenium levels in these samples and that of seawater in order to generate their projections. During much of the last 550 million years, the projected selenium levels in seawater were roughly consistent with today’s levels, except during the time of the three mass extinction events.
Both too much selenium and too little can harm us, yet Earth today seems fine-tuned to provide the just-right amount for life. Recall the words of Psalm 104:29–30: