| Press Images
Scanning electron microscope image of a modern human head
louse, Pediculus humanus.
To
download high-resolution click here:
Credit: Vincent S. Smith, University of Glasgow |
Conventional microscope image of the exoskeleton and some
body parts of the modern human head louse, Pediculus humanus.
To
download high-resolution click here:
Credit: Vincent S. Smith, University of Glasgow |
Methods to control head lice have not changed much over
the centuries, as seen by the 6th century nit comb from
Egypt (upper left) and its modern plastic counterpart (lower
left). At right is a microscope image showing the exoskeleton
and some body parts of a modern human head louse.
To
download high-resolution click here:
Credit: Vincent S. Smith, University of Glasgow for louse
and modern comb photo. Photograph of Egyptian comb courtesy
of Museum of New Zealand Te Papa Tongarewa, Wellington,
New Zealand.
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Oct. 4, 2004 – A University of Utah study
showing how lice evolved with the people they infested reveals
that a now-extinct species of early human came into direct contact
with our species about 25,000 years ago and spread the parasites
to our ancestors.
The study found modern humans have two genetically distinct types
of head lice. One type is found worldwide and evolved on the ancestors
of our species, Homo sapiens. The second type is found
only in the Americas, evolved on another early human species (possibly
Homo erectus) and jumped to Homo sapiens during
fights, sex, sharing of clothes or perhaps cannibalism.
“We’ve discovered the ‘smoking louse’
that reveals direct contact between two early species of humans,”
probably in Asia about 25,000 to 30,000 years ago, says study
leader Dale Clayton, a professor of biology at the University
of Utah. “Kids today have head lice that evolved on two
species of cavemen. One species led to us. The other species went
extinct.”
Alan Rogers, a co-author of the study and professor of anthropology
at the University of Utah, says: “The record of our past
is written in our parasites.”
The analysis of lice genes also confirmed two other key developments
in human evolution. First, it verified studies showing how and
when various species branched off the family tree of primates
and humans. Second, it confirmed the “out of Africa”
theory that the population of Homo sapiens mushroomed
after a small band of the early humans left Africa sometime between
150,000 and 50,000 years ago.
The study will be published online Oct. 5 in the Public Library
of Science journal PLoS Biology. The study’s first
author is former University of Utah postdoctoral fellow David
L. Reed, now assistant curator of mammals at the University of
Florida’s Florida Museum of Natural History. Other authors
are Vincent Smith of Scotland’s University of Glasgow, and
Shaless Hammond, who worked in Clayton’s lab as a high school
student.
Did Modern Humans Date Other Species
– or Kill Them?
Transmission of the second type of lice from a now-extinct human
species to Homo sapiens may have happened during mating,
so Reed plans a study of pubic or crab lice – which only
spread sexually – to confirm or disprove that possibility.
Clayton and Rogers say it’s also possible our ancestors
got the second kind of head lice by fighting with or cannibalizing
another human species – or by sharing or stealing their
clothing.
Clayton says evidence of contact between two species of humans
is surprising because “Homo erectus has long been
thought to have gone extinct hundreds of thousands of years ago,”
although recent studies suggested Homo sapiens might
have had contact with Homo erectus in Asia 50,000 years
ago.
Reed says: “Not only did modern humans live contemporaneously
with close cousins such as Neanderthals, but also with more archaic
hominids such as Homo erectus, a species that we have
not shared a common ancestor with for over a million years. It
is amazing to know that we had physical contact with another species
of human. We either battled with them, or lived with them or even
mated with them. Regardless, we touched them, and that is pretty
dramatic to think about.”
Reed wonders if contact with our species proved fatal.
“When scientists first determined that we (Homo sapiens)
were contemporaneous with Neanderthals (Homo neanderthalensis)
in Europe, it was suspicious that our contact with them immediately
preceded their extinction,” Reed says. “Our study
has provided evidence that we had contact with Homo erectus
in Asia just prior to the extinction of that species as well.
Did we cause the extinction of two other species of humans?”
Findings Show Lice and Different Human Species Evolved
Together
Our genes reveal the evolutionary history only of modern humans.
Fossil evidence is scant for now-extinct species of early humans.
Because lice evolved in concert with the humans they infested,
lice “have recorded events in human evolutionary history
in their DNA,” Reed says.
The researchers analyzed the physical appearance and genetic material
(mitochondrial DNA) of modern human head lice, Pediculus humanus,
to construct a family tree for lice showing when various species
branched off from each other. Genes of modern lice also were used
to reconstruct their population histories over time.
The researchers found the family tree of the lice closely mirrors
the previously published family tree of humans and their primate
ancestors. That was consistent with the well-known phenomenon
that any single species or lineage of lice (like other parasites)
tends to stick only to one species of host and rarely jumps to
other hosts.
Scientists already knew that early ancestors of our species, Homo
sapiens, diverged from other archaic humans about 1.2 million
years ago. (There is semantic debate over whether those archaic
humans should be called Homo erectus, or whether the
name should be reserved for their more recent descendants.) The
new study showed two almost identical-looking but genetically
different strains of head lice diverged 1.18 million years ago.
That indicates each of the two kinds of head lice infested a different
species of early human as the human species diverged.
Genes from both types of head lice are found on people today,
suggesting that after infesting Homo erectus or another
archaic human species for 1 million years, the second louse type
jumped from that soon-to-be-extinct species and onto Homo
sapiens.
“In order for the archaic human lice to still exist on modern
humans, archaic and modern humans had to coexist in time and space,”
Clayton says.
What Lice Say About Theories of Human Evolution
Some of the findings conflict with two major theories of human
evolution – the “replacement model” and “multiregional
model” and instead fit best with a third theory known as
the “diffusion wave model.”
(1) The replacement model says that after primitive human ancestors
first left Africa about 2 million years ago, a second wave spread
out from Africa sometime after 150,000 years ago and certainly
by 50,000 years ago, and then replaced other now-extinct species
of early humans in Africa, Asia and Europe without breeding with
them.
Clayton says that model doesn’t fit the louse data because
if Homo sapiens from Africa replaced archaic humans elsewhere
without interacting with them, the type of lice on archaic humans
would have gone extinct with their hosts instead of jumping to
modern humans.
(2) The multiregional model says early humans from Africa and
elsewhere in the world mated with other each other, so Homo
sapiens gradually evolved in many regions worldwide. But
if so much interbreeding occurred, the two groups of lice probably
would not have remained genetically distinct for the last 1.18
million years, Rogers says.
(3) The diffusion wave model falls between the other two theories.
Like the replacement theory, it says modern humans arose in Africa
and spread across the world, Rogers says. Like the multiregional
theory, it says those early humans mated with humans elsewhere.
The diffusion wave theory adds a new twist, namely, that the genes
of humans spreading from Africa came to dominate the modern human
genetic blueprint because when they mated with archaic humans,
the children were less fit.
“As they come out of Africa, they replace other populations
while interbreeding with them,” Clayton says.
The findings in lice are most consistent with the diffusion wave
hypothesis, which allows some interbreeding among various forms
of early humans but also says the genes of early humans who left
Africa came to dominate Homo sapiens, he adds.
Lice Genes Confirm Key Events in Human Evolution
The new study confirmed several events in primate and human evolution.
The researchers found chimp lice and human lice diverged roughly
5.6 million years ago, consistent with previous evidence that
chimps and human ancestors diverged from a common ancestor about
5.5 million years ago.
The study also supports the controversial view that there was
a “bottleneck” or reduction in the global Homo
sapiens population to only about 10,000 people about 100,000
to 50,000 years ago. Rogers and others have proposed the bottleneck
may have occurred because of a mass die-off of early humans due
to a globally catastrophic volcanic eruption. Others believe the
population bottleneck seen in human genes happened because only
a small group of human ancestors left Africa in the second wave
150,000 to 50,000 years ago, then reproduced to cause a sudden
population expansion.
The new study used the mutation rate in lice and comparisons of
genetic differences among lice to find a similar population bottleneck
in the group of head lice that infested early Homo sapiens,
but no such bottleneck in the population of the lice on the archaic
human species. That means archaic humans didn’t go through
the same population shrinkage and thus must have spread their
lice to Homo sapiens sometime after 50,000 years ago.
Rogers speculates contact occurred 25,000 or 30,000 years ago.
The findings provide independent confirmation of the second “out
of Africa” event because genetic analysis shows the population
of lice – like their Homo sapiens hosts –
also dramatically expanded after the bottleneck.
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