19 October 2019 | New Scientist | 17Origin of life Evolution
Michael Marshall Michael Marshall
WHEN the first life emerged on
Earth, it may have had a helping
hand from an unexpected source:
urea, a chemical found in urine.
The urea may have been used
to make the first simple genes.
Life on Earth began at least
3.5 billion years ago. Nobody
knows exactly how, but it is likely
that one of the most crucial steps
was the formation of the first genes.
Today, most organisms store their
genes on DNA, but this is such
a complex molecule that many
scientists believe life must have
begun with something simpler.
“We argue, let’s start with just two
molecules, formaldehyde and urea,”
says Thomas Carell at the Ludwig
Maximilian University of Munich in
Germany. Both are simple and are
likely to have existed on early Earth.
Carell and his team had already
shown that formaldehyde can be
converted into sugars, including
ribose – a key component of a
DNA-like molecule called RNA –
so they then focused on urea.
The researchers knew that simply
heating urea causes individual urea
molecules to link up into pairs and
triplets. They mixed these urea-
based molecules with ribose and
water, heated them to 95°C until
the mixture dried out, then added
more water. This simulated a
volcanic pond evaporating in
the sun, then filling up again.
The result was molecules similar
to nucleosides, another component
of RNA. Follow-up experiments
showed that they could be inserted
into RNA molecules in place of the
normal nucleosides. Crucially, the
nucleosides in RNA can pair up with
each other and the urea nucleosides
could also do this, suggesting they
could be used as genes for storing
genetic information (Angewandte
Chemie, doi.org/dchm).
“It’s certainly a molecule that
we could consider as an ancestor
of RNA,” says Carell. ❚
Early life may have
had genes made
from urine chemical
OUR mammal ancestors
were cold-blooded for tens
of millions of years after their
first appearance. In this respect,
they remained similar to the
cold-blooded reptiles from
which they evolved.
The finding comes from an
analysis of fossils of two early
mammal-like species, which
suggests the animals lived
relatively long lives and had
slow metabolisms – both
reptile-like traits.
The first mammals evolved
during the dinosaur era. By
the middle of the Jurassic,
around 170 million years ago,
they were common. However,
we don’t know if they were
warm-blooded, or endothermic,
because this trait leaves no
obvious traces behind in the
fossil record.
To find out when warm-
bloodedness evolved, Elis
Newham at the University
of Bristol in the UK and
his colleagues studied two
animals from 200 million
years ago in the Early Jurassic,
Morganucodon and
Kuehneotherium. Both were
mammaliaforms, meaning
they may have been related tomammals without technically
belonging to the group.
Newham’s team studied the
roots of the animals’ teeth. The
roots had a hard coating called
cementum, as ours do. New
layers were added as the animal
aged, so counting the layers
gave an estimate of its age – a
bit like counting tree rings.
Based on 61 specimens, the
researchers estimated that
Morganucodon could live for
14 years and Kuehneotheriumfor nine. That is a long time for a
small mammal: mice rarely live
more than three years. However,
it isn’t unusual for a small
reptile like a lizard. It indicates
that the animals had slow
metabolisms, suggesting they
were cold-blooded, as mammals
need fast metabolisms to
maintain their body heat.
The researchers also
examined fossil thigh bones
belonging to Morganucodon.
By comparing the length of the
bones to the area of openingsinside them through which
blood vessels once flowed, they
could conclude that the animal
had a low rate of blood flow
through its thigh bones. This,
too, suggests a slow metabolism
and cold-bloodedness (bioRxiv,
doi. org/dchk).
The findings fit with other
studies of protomammals, says
Rachel Wallace at the University
of Texas at Austin. “There really
isn’t good indirect evidence for
endothermy until more recent
mammalian taxa, where actual
fur is preserved.” The oldest
mammal known to have fur is
the beaver-like Castorocauda
from the Middle Jurassic,
about 164 million years ago.
The new finding is
striking because, in other
respects, Morganucodon
and Kuehneotherium were
mammal-like, says Eva
Hoffman at Harvard University.
For instance, Morganucodon
had both a big brain and a
modern-looking skeleton.
“That these advanced
mammaliaforms still lived more
like reptiles than like mammals
reflects a mosaic pattern of
evolution, in which some
ancestral features are retained
even as novel features evolve,”
she says.
Hoffman says the
protomammals may have been
similar to the most primitive
living mammals: monotremes,
such as the duck-billed platypus.
Unlike other mammals,
these animals still lay eggs.
“Monotremes have pretty long
lifespans and relatively low
metabolic rates,” she says. There
is also evidence that they aren’t
fully warm-blooded. ❚Ancestors of mammals
retained cold blood
MAGDALENA REHOVA/ALAMY STOCK PHOTOMorganucodon might
have been a cold-
blooded creature14
Morganucodon’s lifespan in years,
suggesting it was cold-blooded