Bacteria Help Early Life on Its First Trip

Purple haze all in my brain
Lately, things just don’t seem the same
Acting funny, but I don’t know why
Excuse me while I kiss the sky

(Jimi Hendrix)

If we time-traveled back 1.8 billion years, the world would certainly not“seem the same”. The apex of life consisted of single-celled slime floating in ancient oceans. Life slowly churned along, but evolution stalled as it contemplated a jump from single-cell clusters to true multicellular plants and animals. Various theories exist about this evolutionary log jam, but recent work suggests that the biosphere was helped along in the Proterozoic by humble little creatures called purple sulfur bacteria.

This storyline started about 3.5 billion years ago when cyanobacteria made their first appearance in Earth’s oceans and learned how to harness the sun’s energy through photosynthesis. A byproduct of their clever scheme was free oxygen. But it took another billion years before oxygen appeared in the atmosphere. One theory is, geological processes sucked up the first billion years of oxygen production to oxygenate Earth’s mantle.

Eventually, cyanobacteria oxygenated the atmosphere, meaning the key ingredient for most animal life today, free oxygen, was available for exploitation by Earth’s evolving life-forms. MIT researchers place this critical date at 2.3 billion years ago. More progress appeared at about 2 billion years when the first eukaryotes appeared, and by 1.1 billion years, multicellular life discovered sexual reproduction. Still, we must move forward to 800 million years before recognizable plants and animals appeared. Why did this process take so long?

The period from 1.8 billion to 800 million years is dubbed the boring billion, reflecting life’s lack of progress on the evolutionary front. Low oxygen levelsare identified in some research as the cause of this evolutionary stall, but a second explanation points to the limited availability of trace elements and nutrients. The limited nutrient aspect of this mystery is where our purple sulfur bacteria enter into the discussion.

Nitrogen

Fans of biology recognize the role nitrogen plays as an essential element for all life. The availability of fixed nitrogen is a limiting factor for plant growth, and nitrogen is critical for protein synthesis in humans. Our genetic code, encapsulated in DNA, requires nitrogen in its four base components adenine, guanine, cytosine and thymine.

Even though nitrogen permeated Earth’s atmosphere 2 billion years ago, this element was out of reach for life processes until specific chemical transitions occurred. Nitrogen fixation is the process of converting pure nitrogen into nitrogen compounds like ammonia, nitrites, and nitrates — creating the reactive forms of nitrogen used by plants and animals for growth and body function.

When limited by the availability of fixed nitrogen, life can’t move forward. Typically scientists consider cyanobacteria as the source of nitrogen-fixing in ancient oceans; however, recent research uncovered another prospective candidate at the bottom of a lake in Switzerland, purple sulfur bacteria. These bacteria exhibit a capacity to fix nitrogen at rates comparable to the low-end of the cyanobacteria fixation range.

Purple Sulfur Bacteria and Ancient Oceans

Researchers speculate that the Swiss Lake where purple bacteria were studied, Lake Cadagno, resembles conditions in ancient Proterozoic oceans with an oxygenated surface layer but anoxic deeper waters. The work also noted low concentrations of molybdenum, a compound necessary for creating the nitrogenase enzymes that bacteria use to fix free nitrogen into usable nitrogen compounds. Perhaps purple sulfur bacteria operated in our ancient oceans, helping life creep forward and ensuring the boring billion years did not turn into the tiresome two billion years.

We have no direct way of knowing the precise conditions in Proterozoic oceans. Continents and oceans have come and gone many times since those boring billion years, and limited evidence remains because Earth’s geological processes churn and recycle ancient rocks.

But the fact remains, evolution stalled for a billion years between atmospheric oxygenation and the appearance of plants and animals in the Ediacaran, immediately before the Cambrian explosion. This period of evolutionary lapse remains an ancient mystery. Still, continuing research slowly sheds light on events that conspired in our dim past and provides us with food for thought about life’s journey on Earth.

A Billion Years of Missing Oxygen (by WM House; Medium)

Before the Explosion (by WM House; Medium)