Could Complex Life Have First Evolved in Lakes?
The Origins of Life
Life began on Earth billions of years ago. The first lifeforms were simple single-celled organisms. After about 3 billion years of life existing only in the single-celled variety, multi-celled animal life appeared about 600 million years ago. From here, animal life diversified very rapidly.
We all know that the original single-celled variety of life, as well as the first multi-celled organisms, first appeared in aquatic environments before moving to land. Conventional wisdom states that these lifeforms first appeared in oceans. Scientists have more recently discovered, however, that the earliest lifeforms can likely be traced back to lakes.
Ancient Lake Bed Fossils Discovered
A team of researchers led by UC Riverside studying rock samples in the ancient lake beds of the Doushanto Formation in the Yangtze Gorges area of South China in 2009 discovered fossils in this ancient lake bed that predate other known early fossils. These newly discovered lake bed fossils are believed to be the oldest animal specimens discovered so far.
Many of the fossils found in this rock formation appear be those of microscopic animal embryos. These rock beds contained almost no adult specimens. It is unclear as to what significance this finding has to the overall research.
The researchers involved in the study collected hundreds of rock samples from several locations throughout South China. During this research, they carried out mineralogical analysis using X-ray diffraction. They also collected and analyzed other types of geochemical data. Lead author of the study, which was published in the July 27-31 2009 online edition of the Proceedings of the National Academy of Sciences, Tom Bristow, had this to say:
"Our first unusual finding in this region was the abundance of a clay mineral called smectite. In rocks of this age, smectite is normally transformed into other types of clay. The smectite in these South China rocks, however, underwent no such transformation and have a special chemistry that, for the smectite to form, requires specific conditions in the water - conditions commonly found in salty, alkaline lakes.
"All our analyses show that the rocks' minerals and geochemistry are not compatible with deposition in seawater. Moreover, we found smectite in only some locations in South China, and not uniformly as one would expect for marine deposits. This was an important indicator that the rocks hosting the fossils were not marine in origin. Taken together, several lines of evidence indicated to us that these early animals lived in a lake environment."
The research concluded that these rock formations were the remnants of ancient lakes, and not ancient oceans. The findings suggest that these fossils are the remains of creatures who lived in a lake environment, and not in a marine environment.
Why this Discovery is Shocking
According to the scientists involved in the research, lakes are relatively short-lived features on the Earth, and environments in lakes are not nearly as consistent as those of oceans. They claim that it is extremely surprising that life could have begun in lakes rather than in oceans. These researchers claim that oceans are much more stable and offer a much more consistent environment in which life could survive and evolve into more complex lifeforms.
Martin Kennedy, a professor of geology in the Department of Earth Sciences who participated in the research stated:
"We know that life in the oceans is very different from life in lakes, and, at least in the modern world, the oceans are far more stable and consistent environments compared to lakes which tend to be short-lived features relative to, say, rates of evolution. Thus, it is surprising that the first evidence of animals we find is associated with lakes, a far more variable environment than the ocean."
The presence of these ancient lake fossils, dated to 600 million years ago, raises questions about what aspects of the Earth's environment changed during this time to enable animal evolution to occur.
The researchers were shocked to find that the oldest fossils found so far came from lake beds, rather than from marine sediments as would commonly be expected based on previous scientific theories. Martin Kennedy went on to state that:
"It is possible, too, that similarly aged or older organisms also existed in marine environments and we have not found them. But at the very least our work shows that the range of early animal habitats was far more expansive than presently assumed and raises the exciting possibility that animal evolution first occurred in lakes and is tied to some environmental aspect unique to lake environments. Furthermore, because lakes are of limited size and not connected to each other, there may have been significant parallel evolution of organisms. Now we must wait and see if similar fossils are found in marine sediments."
The researchers involved in this study have not ruled out the possibility of life having evolved in marine environments concurrently with the life found in the ancient lake beds. They will continue to search for more evidence of life in ocean sediments to see if there are similarly-aged fossils to be discovered there.
Is this Really So Surprising?
I don't find it particularly surprising to think that multi-celled animals may have first evolved in lake environments rather than in oceans. If lake environments are ever-changing, animals will need to adapt and evolve to survive in these ever-changing conditions. When the lakes containing animal life began to dry up, or their conditions began to otherwise change, the animals living in them must evolve quickly to adapt to their new environment. The animals that didn’t adapt quick enough would have gone extinct much more quickly than the ones that evolved to survive in the changing conditions. The lifeforms living in a stable environment like the ocean have less of a need to evolve and adapt to other environments than the creatures living in the less stable environments of a lake.
The early lifeforms that lived in the unstable lake environments had to evolve to handle the fluctuations and changes in their environment and became much more resilient as a result. Any ocean-dwelling lifeforms that may have existed would not have had to evolve or adapt, so they would of course remain the same and take much longer to evolve.
Fossils of similarly-aged single-celled organisms will likely be found in a broader range of environments. The fluctuating conditions of lake environments likely gave rise to more complex and adaptable organisms sooner than the more stable ocean environments.
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© 2018 Jennifer Wilber