New tests confirm antiquity of ancient human footprints in New Mexico
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[October 06, 2023]
By Will Dunham
WASHINGTON (Reuters) - Humans trod the landscape of North America
thousands of years earlier than previously thought, according to new
research that confirms the antiquity of fossilized footprints at White
Sands National Park in New Mexico using two further dating approaches.
The footprints date to about 21,000 to 23,000 years ago, based on
radiocarbon and optically stimulated luminescence dating techniques,
researchers said on Thursday, showing that our species Homo sapiens
already had a foothold in North America during the most-inhospitable
conditions of the last Ice Age.
Massive ice sheets covered wide swathes of the continent - reaching as
far south as Illinois - amid extensive glaciation.
A 2021 study by these researchers also dated the footprints, based on
tiny plant seeds embedded in the sediment alongside them, to about
21,000 to 23,000 years ago. This was met with skepticism from some
scientists who questioned the dating conclusion.
"Every dating technique has strengths and weaknesses, but when three
different techniques all converge on the same age range, then the
resulting ages are exceptionally robust," said Jeff Pigati, a research
geologist at the U.S. Geological Survey (USGS) in Denver and co-lead
author of the research published in the journal Science.
"Our original results were controversial, and we knew all along that we
needed to independently evaluate the seed ages to develop community
confidence in them. This paper is that corroborative exercise," added
study co-lead author Kathleen Springer, also a USGS research geologist
in Denver.
Homo sapiens arose in Africa more than 300,000 years ago and later
spread worldwide. Scientists believe our species entered North America
from Asia by trekking across a land bridge that once connected Siberia
to Alaska.
Previous archaeological evidence had suggested that human occupation of
North America started roughly 16,000 years ago, according to study
co-author Matthew Bennett, a professor of environmental and geographical
sciences at Bournemouth University in England.
"Indigenous peoples were there earlier than thought, before the great
ice barrier at the height of the last glacial maximum closed the way
south from Alaska. By what route and how they got there is yet to be
determined. White Sands is just one point on the map for now," Bennett
said.
The 2021 study dated the footprints using radiocarbon dating to
determine the age of seeds of a common aquatic plant called spiral
ditchgrass found alongside the fossilized foot impressions.
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Ancient human footprints at White Sands National Park in New Mexico,
U.S., are seen in this undated handout photograph obtained by
Reuters on October 5, 2023. U.S. National Park Service/Handout via
REUTERS
The technique is used to determine the age of organic material as
old as roughly 60,000 years based on the decay of an isotope called
carbon-14, a variant of the element carbon. Living organisms absorb
carbon-14 into their tissue. After an organism dies, this isotope
changes into other atoms over time, providing a metric for
determining age.
The fact that aquatic plants can acquire carbon from dissolved
carbon atoms in the water, potentially throwing off a dating
estimate, caused controversy over the 2021 conclusions.
This time, the researchers used radiocarbon dating on conifer
pollen, avoiding any concerns about aquatic plants. They isolated
thousands of conifer pollen grains from the same sediment layers as
the ditchgrass seeds. The pollen age statistically matched the seed
age.
The researchers also used optically stimulated luminescence dating
to determine the age of quartz grains within the footprint-bearing
sediments. This method establishes an object's age by measuring the
amount of energy it has trapped since being buried. It found that
the footprint-bearing sediment layers had a minimum age of about
21,500 years.
"The work confirms the chronology we set out in 2021 for the site
using independent methods, labs and approaches," Bennett said.
The footprints - 61 in total - were found at what was a lakeshore.
"People walked on a mosaic of wet and dry ground. There was mud,
silt and sand in this lake-edge environment," Pigati said.
"And just like today, if anyone walks in a similar setting, their
footprints are preserved if they are covered with another layer of
sediment," Springer added.
(Reporting by Will Dunham, Editing by Rosalba O'Brien)
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