Unos científicos afirman haber encontrado microfósiles de hace

EL PAIS, 22 de agosto de 2011
Unos científicos afirman haber encontrado microfósiles de hace
3.400 millones de años
Podrían ser los restos de microbios más antiguos identificados hasta ahora
Estructuras tubulares que son
microfósiles, según los científicos que
los han descubierto en rocas de
Australia de hace 3.400 millones de
años.- DAVID WACEY
A.R. - Madrid | Un equipo de
científicos de Australia y del
Reino Unido afirma haber
descubierto unos microfósiles de
hace 3.400 millones de años,
restos de unos organismos
unicelulares
que
utilizarían
compuesto de azufre para
obtener energía. Se trata de unas
estructuras halladas en rocas de
Australia occidental que, según
David Wacey y sus colegas, son
los microfósiles más antiguos que se conocen. Sin embargo, la revista Nature
Geoscience, en la que se da a conocer el trabajo, no hace mención alguna en la
presentación del mismo a esa condición de "más antiguos". Los debates científicos en
la búsqueda de los restos de microbios remotos, siempre rondando una antigüedad
superior a los 3.000 millones de años (la edad de la Tierra es de unos 4.500 millones),
tiene ya varios años y ahora se impone la prudencia entre los investigadores, ya que ha
habido hallazgos previos de estructuras que pudieran ser restos celulares del pasado
que han sido luego desmentidos o al menos
seriamente cuestionados en sucesivos
análisis.
En la cresta de Strelley Pool, en Australia occidental,
se han descubierto rocas con unas estructuras que
son microfósiles de hace 3.400 millones de años,
según los científicos.- DAVID WACEY
Wacey (Universidad de Australia Occidental)
y sus colegas han analizado las
microestructuras que han descubierto en las
rocas -en la formación Strelley Pool- y
explican que están asociadas a minúsculos
cristales de pirita (mineral compuesto de
hierro y azufre) y que la composición de
azufre sugiere que la pirita se formó como producto derivado del metabolismo celular.
"Finalmente tenemos pruebas sólidas de vida hace 3,4 millones de años que confirman
que había bacterias entonces viviendo sin oxígeno", afirma uno de los investigadores,
Martin Brasier, de la Universidad de Oxford (Reino Unido) en un comunicado de dicha
institución, del que, sin embargo, se desvinculan expresamente los responsables de
Nature Geoscience.
Los microfósiles están bien conservados entre los granos de arena de una de las más
antiguas rocas sedimentarias que se han encontrado en la Tierra, explican los
científicos. La formación de dichas rocas entre dos episodios volcánicos ha ayudado a
datar con exactitud su antigüedad. Las estructuras celulares tienen tamaño similar y
parecen estar formando grupos.
El récord de antigüedad de microfósiles estuvo durante un tiempo en unas estructuras
algo más antiguas que estas, de hace 3.465 millones de años, descubiertas, también en
Australia Occidental, por el estadounidense J.William Schopf (Universidad de California
en Los Ángeles) en 1993, recuerda The New York Times. Pero en 2002, el hallazgo fue
cuestionado cuando Brasier afirmó que esas estructuras no eran de origen biológico
sino productos minerales. Brasier es uno de los líderes del equipo que reivindica ahora
los fósiles más antiguos de microbios, mientras que Schopf sigue defendiendo su
descubrimiento y está trabajando para demostrar su validez.
Brasier, Wacey y sus colegas afirman que sus análisis y resultados son más
concluyentes que los de estudios precedentes. Sin duda el escrutinio de los
especialistas y de sus competidores será intenso.
http://www.elpais.com/articulo/sociedad/cientificos/afirman/haber/encontrado/microfosiles/hace/3400/millones/
anos/elpepusoccie/20110822elpepusoc_7/Tes
Science News, Aug. 22, 2011
Oldest Fossils On Earth Discovered
Earth's oldest fossils have been found in Australia by a team from the
University of Western Australia and Oxford University. The microscopic
fossils show convincing evidence for cells and bacteria living in an oxygenfree world over 3.4 billion1 years ago.
Fossilized cells from 3.4 billion years ago. (Credit: David Wacey)
The team, led by Dr David Wacey of the University of Western Australia and including
Professor Martin Brasier of Oxford University, report the finding in the journal Nature
Geoscience.
'At last we have good solid evidence for life over 3.4 billion years ago. It confirms there
were bacteria at this time, living without oxygen,' says Professor Brasier of the
Department of Earth Sciences at Oxford.
Earth was still a hot, violent place at this time, with volcanic activity dominating the
early Earth. The sky was cloudy and grey, keeping the heat in despite the sun being
weaker than today. The water temperature of the oceans was much higher at 40-50
degrees -- the temperature of a hot bath -- and circulating currents were very strong.
1
9
12
Note: In Europe, 1 billion years represents 1,000,000,000 years (10 years), 1,000,000,000,000, (10 )
than for Americans are a trillion.
Any land masses were small, or about the size of Caribbean islands, and the tidal range
was huge.
Significantly, there was very little oxygen present as there were no plants or algae yet
to photosynthesise and produce oxygen. The new evidence points to early life being
sulfur-based, living off and metabolizing compounds containing sulfur rather than
oxygen for energy and growth.
'Such bacteria are still common today. sulfur bacteria are found in smelly ditches, soil,
hot springs, hydrothermal vents -- anywhere where there's little free oxygen and they
can live off organic matter,' explains Professor Brasier.
The microfossils were found in a remote part of Western Australia called Strelley Pool.
They are very well preserved between the quartz sand grains of the oldest beach or
shoreline known on Earth, in some of the oldest sedimentary rocks that can be found
anywhere.
'We can be very sure about the age as the rocks were formed between two volcanic
successions that narrow the possible age down to a few tens of millions of years,' says
Professor Brasier. 'That's very accurate indeed when the rocks are 3.4 billion years old.'
The microfossils satisfy three crucial tests that the forms seen in the rocks are
biological and have not occurred through some mineralization process.
The fossils are very clearly preserved showing precise cell-like structures all of a similar
size. They look like well known but much newer microfossils from 2 billion years ago,
and are not odd or strained in shape.
The fossils suggest biological-like behavior. The cells are clustered in groups, are only
present in appropriate habitats and are found attached to sand grains.
And crucially, they show biological metabolisms. The chemical make-up of the tiny
fossilized structures is right, and crystals of pyrite (fool's gold) associated with the
microfossils are very likely to be by-products of the sulfur metabolism of these ancient
cells and bacteria.
Early fossils of life on Earth has been a controversial area. In the past decade, the
barriers that need to be overcome before claiming such evidence have been raised
significantly, aided by new techniques for mapping the chemistry of rocks at fine
scales.
In 2002, the same Oxford group suggested well-known microfossils from the Apex
chert in Australia were not the preserved forms of ancient bacteria after all. They
argued that the context, shape and mineralogy of the forms were all wrong for them
to be of biological origin.
They believe the current fossils, found just 20 miles away, satisfy all criteria for judging
such finds.
The researchers are now using the techniques and approaches they used in this study
to re-examine other fossil finds that have been proposed to contain evidence for life
on Earth at these extremely early times.
'We're now making detailed comparisons with all other early microfossils, and we're
very optimistic for future finds,' says Professor Brasier.
The work also has implications for looking for life on other planets, giving an indication
of what evidence for such life might look like.
Should there be life elsewhere in our solar system -- on Mars or on the moons of Titan
or Europa -- it is likely to be similar sorts of bacteria and cells living in similar
environments. So any fossils in rocks from these planets and moons ought to look like
these Australian microfossils and pass the same evidence tests.
'Could these sorts of things exist on Mars? It's just about conceivable,' says Professor
Brasier. 'But it would need these approaches -- mapping the chemistry of any
microfossils in fine detail and convincing three-dimensional images -- to support any
evidence for life on Mars.'
http://www.sciencedaily.com/releases/2011/08/110821205241.htm