Are we alone? The elusive answer to that profound question may be coming soon as astronomers search for the holy grail of signs of life on worlds that circle stars beyond our Sun. So far, Earth is the only world in the entire unimaginably vast Universe that is actually known to nurture living creatures. Of course, this does not mean that just because scientists haven’t found extraterrestrial life yet–and there is no convincing evidence that it has found us–that it does not exist. In July 2019, astronomers from Cornell University in Ithaca, New York, announced that they have dipped into Mother Nature’s mysterious paint box, that was used to tint the ancient Earth, in order to create a cosmic “cheat sheet” in their quest to discover the evolution of life as we know it on distant worlds. By correlating hues and tints, the scientists hope to understand where discovered exoplanets may reasonably fall along their own evolutionary spectrum.
“In our search to understand exoplanets, we’re using the early Earth and its biological milestones in history as a Rosetta stone,” commented Dr. Jack O’Malley-James in a July 10, 2019 Cornell University Press Release. Dr. O’Malley-James, a research associate at Cornell’s Carl Sagan Institute, recently co-authored a paper with Dr. Lisa Kaltenegger, a professor of astronomy and director of the Sagan Institute. The research paper was published in the July 9, 2019 issue of the Astrophysical Journal Letters under the title Expanding the Timeline for Earth’s Photosynthetic Red Edge Biosignature.
Even though life has not been discovered on worlds other than Earth, there is a great statistical probability that it exists throughout the Cosmos–and that it is likely to be quite abundant. One of the most basic, as well as least resolved, biological questions is the answer to how life on Earth originated. Indeed, in order to come to a scientific understanding of what life may be like on other worlds, it is of paramount importance to understand how life developed on Earth–where it originated, how it has evolved, and where it exists. Earth is literally the only laboratory, that scientists have access to, where they can study the formation of life.
All life on our planet is based on the element carbon. Despite this, living organisms on Earth are extremely diverse, leading to the unavoidable conclusion that whatever life may exist throughout the Cosmos–be it carbon-based or based on an alternative biochemistry–will likely be a complicated collection of exotic life-forms. The life that may dwell on distant worlds could be remarkably wise and aware–or just as devoid of consciousness as a pine tree or a rose. Because living creatures on other worlds followed an entirely different line of evolution, they will not look like us. For example, some animal species on Earth do not have faces. There is no reason to assume that a creature from another world will have a face.
Faces are not uniquely human, even though with their subtle degrees of expression and symmetry, they have set standards for both human beauty and tragedy, in both art and drama. Human beings evolved their faces along with all the other animals that have back bones (vertebrates), from catfish to cats.
Considering the complicated history of life on our planet, its multiple beginnings and extinctions; its accidental detours and various migrations, it would seem rather unlikely that evolution on other worlds would follow precisely the same course as life on Earth. Indeed, it could happen that we might not even recognize life from another world if we saw it. This is because it may exist in a form that is unfamiliar to us.
Even on our own planet, bizarre forms of life exist–and in the strangest places. Extremophiles are living creatures that exist on Earth under conditions that human beings would find hostile–and that would kill us, as well as many other creatures on our planet. The existence of these weird creatures was first recognized back in the 1970s, and the more scientists investigated these Earthly oddities, the more odd species they discovered. Scientists have found that most archaea, bacteria, and a handful of protists, can survive quite easily in harsh environments. However, even though the majority of extremophiles are microbes, there are exceptions.
A Basic Timeline For Life On Earth
Ever since Earth first emerged from its natal protoplanetary accretion disk— composed of gas and dust that swirled around our ancient Sun–it has changed many times over. Our planet has not always been hospitable to life, and it has also suffered a series of mass extinctions. One especially violent collision between the protoEarth and a Mars-sized protoplanet that scientists have named Theia, is responsible for the birth of Earth’s lovely Moon–and without our Moon, life would not have emerged and evolved on Earth. When the doomed Theia struck our ancient planet, in the early days of our Solar System, it caused myriad moonlets to be shot screaming into the sky above our planet. This debris, composed of both Earth-stuff and Theia-stuff, eventually coagulated to form Earth’s bewitching lunar companion. Another devastating event occurred much later–about 65 million years ago–when a large comet struck our planet. This violent event is thought to have contributed to the demise of the dinosaurs, which enabled mammals to achieve dominance.
The fossil record indicates that many organisms that thrived long ago are now extinct. Indeed, 99% of the species that have ever lived on Earth no longer exist.
Our Solar System is about 4.6 billion years old, and our planet is between 4.4 and 4.57 billion years old. Life on Earth first emerged from non-living substances about 3.5 to 4 billion years ago, and the first life forms were unicellular organisms similar to bacteria.
The basic timeline for the evolution of life on Earth includes the following:
-Approximately 3.5-3.8 billion years ago, simple cells called prokaryotes thrived on our young planet.
-About 3 billion years ago, photosynthesis developed.
-About 2 billion years ago complex cells called eukaryotes emerged.
-Approximately 1 billion years ago multicellular life evolved.
-About 600 million years ago simple animals emerged on Earth.
-Approximately 570 million years ago arthropods evolved. They were the ancestors of insects, arachnids, and crustaceans.
-550 million years ago, complex animals evolved.
-500 million years ago, fish and proto-amphibians formed in the waters of Earth.
-475 million years ago, land plants evolved.
-400 million years ago insects and seeds emerged.
-360 million years ago amphibians–such as frogs–evolved.
-300 million years ago, reptiles evolved and thrived.
-200 million years ago, the first small mammals emerged on Earth.
-150 million years ago, birds evolved from their avian dinosaur ancestors.
-130 million years ago, flowers–like roses–formed on Earth.
-65 million years ago, the non-avian dinosaurs died out. A crashing comet likely delivered the final blow to the formerly dominant reptiles. This paved the way for mammals to evolve, and eventually become dominant on Earth.
-2.5 million years ago, the ancestors of human beings appeared.
-200,000 years ago, modern humans emerged.
-25,000 years ago, neanderthals died out. The neanderthals and modern humans co-existed for a time, and it is likely that both are the ancestors of humans living today.
Mother Nature’s Paint Box
“If an alien had used color to observe if our Earth had life, that alien would see very different colors throughout our planet’s history–going back billions of years–when different life forms dominated Earth’s surface,” Dr. Lisa Kaltenegger noted in the July 10, 2019 Cornell Press Release.
“Astronomers had concentrated only on vegetation before, but with a better color palette, researchers can now look beyond a half-billion years and up to 2.5 billion years back on Earth’s history to match like periods on exoplanets,” she continued to explain.
For approximately 10% of Earth’s “lifetime”–that is, for the last half-billion years–the chlorophyll that exists in numerous familiar species of flora has been an important component of our planet’s biosignature. However, other flora, such as algae and cyanobacteria, are considerably older than land-based vegetation. Nevertheless, their chlorophyll-containing structures display their own tattle-tale signatures on Earth’s surface.
“Scientists can observe surface biosignatures beyond vegetation on Earth-like exoplanets by using our own planet as the key for what to look for,” Dr. O’Malley-Jones noted in the July 10, 2019 Cornell University Press Release.
Dr. Kaltenegger explained in the same Press Release that “When we discover an exoplanet, this research gives us a much wider range to look back in time. We extend the time that we can find surface biota from 500 million years (widespread land vegetation) to about 1 billion years ago with lichen and up to 2 or 3 billion years ago with cyanobacteria.”
Dr. O’Malley-Jones and Dr. Kaltenegger model spectra of Earth-like exoplanets–in their search for life as we know it–using different surface organisms that use chlorophyll. Their models might include where a few organisms dominate the entire surface of an Earthlike exoplanet. Imagine the swampy, fictional planet named Dagobah–the home of Yoda–in the Star Wars movies.
Lichens are a symbiotic fungal and photosynthetic partnership–meaning that these two different species live together in harmony, help one another survive, and at first glance seem to be a single species. Lichens may have colonized our planet’s land masses abut 1.2 billion years ago, and their existence would have painted Earth in sage to mint green hues. This type of coating would have generated a “nonvegetative” photosynthetic red-edge tattle-tale signature (the part of the spectrum that protects plants from being burned by our Sun) before the modern biota of today’s Earth became dominant.
Dr. O’Malley-James and Dr. Kaltenegger noted that cyanobacteria, such as surface algae, may have been widespread between 2 and 3 billion years ago on Earth. This would have produced a photosynthetic red edge, which could be found on other distant, Earth-like exoplanets that may potentially host life similar to that on our own planet when it was young.
This important new study shows that lichens, algae and cynanobacteria could have produced a detectable surface red edge feature for an early Earth–long before land vegetation became widespread about 500 million to 750 million years ago, noted Dr. O’Malley-James.
“This paper expands the use of a photosynthetic red edge surface bio-feature to earlier times in Earth’s history, as well as to a wider range of habitable extrasolar planet scenarios,” he added.