UCSB earth science professor emeritus James Kennett and 15 other researchers discovered 13,000-year-old black mat sedimentary deposits believed to be evidence of Earth’s encounters with extraterrestrial materials.

The researchers gathered nanodiamond sediments that are unique cosmic collisions from the bottom of Lake Cuitzeo in central Mexico, supporting claims that our planet underwent a galactic rendezvous 12,900 years ago. The team of international scientists, led by Isabel Israde-Alcántara of Mexico’s Universidad Michoacana de San Nicólas de Hidalgo, published their findings in the Proceedings of the National Academy of Sciences journal on March 5.

The period that followed the suspected comet or asteroid impact was marked by climate change and the extinction of large animals, such as the saber-toothed cat and the mammoth and the decline of prehistoric Indian populations, according to Marine Science Institute Associate Researcher Geophysicist Craig Nicholson.

“What is really interesting about this hypothesis if it is true — and there is more and more evidence now [to support it] — [is that] this would be the first case where extraterrestrial impact actually affected human evolution,” Nicholson said. “If this impact had not occurred, the climate may not have changed, the Clovis culture may have evolved and Native Americans may have had animals to domesticate.”

The discovery marks the latest site in a series of similar findings across North America and Europe, widening the geographic range of impact to the tropics.

Santa Barbara Museum of Natural History Curator of Anthropology John Johnson said his team first discovered similar sediments last decade during a major excavation near Arlington Springs on Santa Rosa Island.

“[A colleague of mine] went back to samples he had obtained in 2001, and when he did he noticed that he had all these little carbon spherules,” Johnson said. “Because we had met Jim Kennett and he had told us about the indicators, we brought that to his attention, so they went back to the location [and found evidence of a cosmic impact].”

Kennett said the evidence that emerged from the dark band of sediments gives scientists a better understanding of outer space’s impact on human populations.

“It is really a paradigm shift in the way we think about cosmic impacts and the effect they can have on earth and life on earth,” Kennett said. “This is the largest major impact that has occurred during the time of our species — [meaning] anatomically modern humans — and [we are arguing] that it had some pretty substantial effects on human culture and population, especially in North America.”

According to Johnson, deposits containing this distinctive mixture of microscopic diamonds and formations of melted rock called spherules have only been found on the Earth’s surface during two other geologic eras.

Despite the growing number of sites with the unique composition, the cosmic impact hypothesis continues to be the topic of controversy. Kennett said he and his colleagues worked diligently in preparation to thwart any potential criticism.

“We did a lot of work that was in response [to critics who] questioned our identification of diamonds, and have probably done more different kinds of analyses than anyone else,” Kennett said. “We refuted the speculation made by some that our identification of diamonds was incorrect.”

The extraterrestrial impact theory accounts for Earth’s geologic and societal evolution in a way that no other argument has been able to fully justify, Johnson said.

“[The hypothesis] explains what is going in world climate, it explains what is going in terms of change in large mammal species that disappeared and changes in the archeological record,” Johnson said. “And people had never thought about this before because we do not think about comet impact.”