Early Diversity

Species Diversity Less Dramatic Than Believed

400 million-year-old trilobite fossil. Trilobites dominated the as a marine fauna during the Cambrian Period.
Credit: NYSED

A study published in the current issue of Science challenges the long-held belief that diversity of marine species has been increasing continuously since the origin of animals. Dr. Thomas D. Olszewski, a geology and geophysics professor at Texas A&M University, has been a part of the international team that carried out this decade-long study, which concludes that most of the diversification occurred early on – relatively speaking. The team’s findings shed new light on the evolution of life over geological timescales on our planet.

"The general understanding for many decades has been that since the rise of the modern major groups of animals about 545 million years ago (i.e., since the beginning of the Phanerozoic Era), the diversity of animal life in the seas has undergone a roughly four-fold exponential increase,” says Olszewski. A steep increase in the diversity was believed to have occurred only between 145 million and 60 million years ago.

Over time, the oceans have expanded and contracted due to the shifting of the Earth’s tectonic plates and changes in climate. Old methods for estimating the historic diversity of life on Earth did not account for the effects of processes like continental drift.
Credit: USGS

But many paleontologists were doubtful about the accuracy of this theory, which was derived using older methods. Olszewski explains that the older methods did not account for many important occurrences in the history of the Earth, including changes in the geography of Earth due to continental drift and variations in the state of global climate.

Collaborative efforts of 35 researchers from the U.S., Germany, the UK, France and Slovakia resulted in a more accurate interpretation of the prehistoric data. Olszewski says that the researchers used a “fundamentally new analysis, which differs in several important aspects from the previous [methods used for] understanding of the history of marine diversity.”

The analysis helped the researchers conclude that the increase in species diversity through the Phanerozoic Era was much less dramatic than previously believed. “Diversity levels comparable to the present day appear to have been reached after a few tens of millions of years following the first appearance of modern animal groups,” says Olszewski.

‘Explosions’ of diversity in the evolution of life were of concern to Charles Darwin, who expected evolution to happen at a constant pace over long periods of time. Today, scientists are beginning to understand the connections between these ‘explosions’ and changes in the Earth’s environment.
Credit: John Collier (portrait) and James Stanfield (photo)

The new fossil data also indicate that the current pattern of distribution of life — with low species diversity in the poles and a very high diversity in the tropics — was established some 450 million years ago.

With the huge amount of data that was used for the analysis, (fossil occurrences representing nearly 3.5 million specimens) it also became possible to assess the diversity changes in local ecological communities as well as in that of the global total. Again, the researchers concluded that local ecological communities have changed relatively little since the establishment of marine animal ecosystems during the Phanerozoic Era.

So what bearing do the study conclusions have on the life on Earth today? Maybe a great deal, Olszewski says.

“As global climatic conditions change, either naturally or anthropogenically, (animal) life responds, which in turn can influence human life,” says Olszewski. “Understanding what life was like under different conditions can help us assess and prepare for the consequences of this ongoing change,” he adds.

Related Web Sites

Astrobiology Roadmap Goal 4: Earth’s Early Biosphere and its Environment
Astrobiology Roadmap Goal 5: Evolution, Environment and Limits of Life
Biological Diversity: Fact or Artifact?
Earth’s Oldest Fossils Reverse Course
Epic Ebbs and Flows
The Avalon Explosion