وجد العلماء بمتحف تاريخ الطبيعه بجامعه كوبنهاجن بالدنمارك
the Natural History Museum of
Denmark at the University
of Copenhagen
حمض نووى DNA
لحفريه أحد الخيول يعود تاريخه
الى 700.000 سنه مضت
مع العلم أن أقدم الأحماض النوويه
كانت للدببه القطبيه وترجع الى 50.000 أو 110.000 سنه
ممايجعل هذا الحصان أقدم حيوان اكتشف حتى الان
المقال كامل بلغه المصدر
DNA shines a light back into
the past, showing us things that fossils can't. But how far back can that light
extend?
Some of the oldest DNA
sequences come from mastodon and polar bear fossils about 50,000 and 110,000
years old, respectively. But a new study published online today in the journal
Nature reports the latest in the push for recovering ever more ancient DNA
sequences. Samples from a horse leg bone more than 700,000 years old have
yielded the oldest full genome known to date.
"We knew that sequencing
ancient genomes as old as 70,000 to 80,000 years old was possible," said
Ludovic Orlando, an evolutionary geneticist with the Natural History Museum of
Denmark at the University
of Copenhagen. "So
we said, why not try even further back in time?"
The Pleistocene horse genome
Orlando and colleagues pieced together helped them determine that the ancestor
to the Equus lineage—the group that gave rise to modern horses, zebras, and
donkeys—arose 4 to 4.5 million years ago, or about two million years earlier
than previously thought. (Learn more about the evolution of horses.)
The ancient horse genome also
allowed the team to determine the evolutionary relationship between modern
domestic horses and the endangered Przewalski's horse, a native to the
Mongolian steppes that represents the last living breed of wild horse.
The team found that
Przewalski's horses were an offshoot of the lineage that gave rise to domestic
horses. The two groups diverged around 50,000 years ago.
Once considered extinct in
the wild, Przewalski's horse was re-introduced into the wild from a captive
population of only a few dozen. While this number suggests that the genetic
diversity of the species might be too small to support, the study shows that
Przewalski's horses are in fact more genetically diverse than domestic breeds
such as Arabian and Icelandic horses.
"We think that there's
enough genetic diversity within the Przewalski's horse to keep conservation
efforts viable," Orlando
said.
Cold Storage
Extracting ancient genomes
from long-dead samples is labor intensive, and there is a limit to how far back
one can go.
Studies on the half-life of
DNA suggest that even under ideal circumstances, DNA sequences older than 1.5
million years will be too short to be readable. So it's highly unlikely that
DNA will be recovered from dinosaurs, since they disappeared 65 million years
ago, except for the lineage leading to modern birds.
But the preservation
environment of an ancient sample can help extend the amount of time DNA has
before it degrades past the point of being recoverable.
"Cold is good," said
Orlando. Frozen
is even better, because liquid water isn't present to degrade DNA molecules.
The six-inch (15-centimeter) horse
leg bone the team analyzed originated in the Yukon
Territory of western Canada. Permafrost kept the remains
in a kind of cold storage for about 735,000 years until scientists dug it out
in 2003.
To determine whether there
might be any biological molecules left in the sample, Orlando and colleagues
first looked to see if they could spot amino acids from collagen—a protein
found in bone—in the specimen.
Once they identified and
successfully sequenced those proteins, the researchers moved on to trying to
extract DNA from the ancient leg bone.
As is the case with the
majority of ancient fossils, most of the DNA they found was from bacteria that
had populated the bone after the horse died. Using DNA from modern horses as a
reference, the team was able to identify "endogenous" DNA that
belonged to the ancient horse itself.
"We sequenced 12 billion
DNA molecules, of which 40 million [were of] horse origin," said Orlando. "There was
a bit of horse DNA in an ocean of microbial DNA."
A New
World
The recovery of a genome
almost an order of magnitude older than any previous genomic information opens
up a wide range of new targets for studying fossils at the genetic level, possibly
including ancient human species, if they lived in cooler environments.
"You name it—what are
your favorite Pleistocene beasts?" wrote Hendrik Poinar, an evolutionary
geneticist at McMaster University in Ontario,
Canada, in an
email.
Poinar, who was not involved
in the current study, would like to see this applied to elephant evolution.
"This should address issues related to the origin of hair and size
plasticity and how they adapted to very different ecologies."
He was not surprised that
researchers were able to sequence a complete genome from 700,000 years ago. It
just takes time and money, Poinar said.
But he also points out that
sequencing ancient genomes is more about preservation in various environments
than the age of a specimen. "I am sure there will be older genomes soon
enough."
