Ancient life, potentially millions of years old and barely alive, found beneath ocean floor

Their ability to survive in such impoverished conditions supports the belief that bacteria could also survive in material ejected into space from a planet by an asteroid impact. Such space-borne microbes could later be deposited, as stowaways on a meteorite, on a different planet.

Scientists in recent years have speculated that life might have originally appeared on Mars when it was warmer and wetter about 4 billion years ago, and then been transported via meteorite to Earth.

A desert in the ocean

The scientific team led by Roy cruised to a spot in the northern Pacific far from the nearest land mass, and thus removed from most wind-borne dust or stream-carried sediments. The scientists picked a place where a current called the North Pacific Gyre inhibits the influx of sediments from far away. This ensured that the sea floor would be essentially a desert, with little organic material raining down.

In effect, the scientists used the Pacific as a laboratory in which they could study millions of years of biological activity by drilling cores roughly 100 feet below the sea floor. The bottom of the sediment cores contained material laid down 86 million years ago.

What they found is the latest example of life’s ingenuity. As expected, there were far more microbes at the top of the sediment column than at the bottom. But even at rock bottom, in red clay that hadn’t been exposed to oxygen or new nutrients — much less sunlight — since the Cretaceous Period, a smattering of bacteria found a way to eke out an existence.

“These communities have not received input or new food since the dinosaurs walked the planet,” Roy said. “Those that are left down there are the ones that can deal with the lowest amount of food.”

An old-age question

The research pivots on the measurement of oxygen in the sediment. By calculating the rate at which oxygen disappears, the scientists can determine the metabolic speed of the organisms.

Key questions remain: How rapidly do these bacteria reproduce? Have they evolved and developed new traits in these eons in the subsea muck, or do they survive thanks to traits that were already there when they first inhabited the sea floor?

And how old are they, exactly?

It’s a big number, the scientists think.

“The notion of being several thousand years old is really a minimum of what is going on,” Hoehler ventured.

One possibility is that, over time, these bacteria have slowly reproduced, and thus today’s population is countless generations removed from the original population. But another idea is that the bacteria are in­cred­ibly old and are the last, hardy remnants of the population that flourished when the sediments first settled on the sea floor.

Living isn’t easy for any organism, and there is wear and tear on cellular structures. Thus any very old bacteria would have had to repair themselves. In theory, they could be millions of years old.

“If we look at how fast they metabolize, it would take them a thousand years just to reproduce themselves. They may be much older than this. There’s no way of knowing,” Roy said.

The slow rate of reproduction means that they cannot evolve at the same speed as bacteria in friendlier, energy-rich, nutrient-thick settings. That means, in turn, that they may preserve more primitive genetic features than other bacteria, says Robert Hazen, a scientist at the Carnegie Institution’s Geophysical Laboratory in the District and author of the new book “The Story of Earth.”

“If we’re looking at deep life, we’re looking at ancient life because they appear to evolve more slowly,” Hazen said. “You might be able to see more deeply rooted aspects of biochemistry. Some of the genes that are present may be just intrinsically much, much older.”

The meaning of life

Hazen has speculated that the definition of life may need to be expanded — that perhaps there are growing films of molecules deep below the surface that have characteristics similar to those of traditional organisms.

The basic definition of life has long been a matter of debate, though life as we know it has a couple of key features.

First, living things have a metabolism, meaning they use chemical reactions to convert energy to useful purposes.

Second, they have a genetic code that prescribes their structure and enables them to reproduce. The inexact nature of reproduction, due to mutations, allows life to undergo Darwinian evolution.

There are other features of living things: They have an inside and an outside. They use liquid water as the medium for their chemistry. And they’re built on a foundation of carbon, which is useful for forming long, complex molecules (such as DNA) that are stable at a variety of temperatures.

The weird microbes in the red clay below the deep Pacific abide by these basic rules of life. They just take their sweet time about it.