¿Cuándo empezó la vida en nuestro planeta?

¿Cuándo empezó la vida en nuestro planeta?

El planeta Tierra tiene una edad aproximada de 4.600 m.a.  Durante varios miles de años estuvo deshabitado, ya que se encontraba en proceso de formación, sometido a una serie de transformaciones que lo prepararían para convertirse, más adelante, en el hogar de miles de millones de organismos vivos.

                Acerca de cómo se originó la vida en nuestro mundo, desde hace  décadas los científicos  barajan diversas teorías tales como la de la Panspermia, según la cual la vida llegó del espacio,contenida en asteroides;  la de Oparin –Haldane, propuesta por el bioquímico ruso Aleksandr Oparín, en  1924, y por el bioquímico y genetista inglés John Haldane, en 1928, según la cual, las primeras células surgieron  a partir de materia orgánica en el agua, de la síntesis abiótica de compuestos que se encuentran en la atmósfera secundaria y por acción de diversas fuentes de energía. Cada uno de ellos pudo demostrar lo que decía a través de experimentos.   Otra hipótesis, más reciente, es la del Mundo de ARN,  que propone que  el ARN fue la primera forma de vida, que eventualmente  evolucionó hacia el mundo de las proteínas ADN.

Vista transversal de un fragmento de roca en el que se pueden ver los estromatolitos de forma cónica de 3.700 millones de años de antigüedad. / Crédito Yuri Amelin

                Existen muchas  más tentativas de explicar cómo surgieron los primeros seres vivos, así como muchas interrogantes al respecto,que poco a poco  podrán responderse a través de investigaciones que vayan aportando las piezas  necesarias para reconstruir  el gran rompecabezas que supone el  origen de la vida en la Tierra.

                De lo que sí podemos estar cada vez más seguros es de cuándo aparecieron los primeros habitantes de este planeta, gracias al registro fósil, que de alguna manera  nos permite atisbar, aunque fuera un poquito, ese misterioso pasado del que algo sabemos, mostrándonos cómo era,en parte,  la vida en aquellas tempranas épocas  de la Tierra.

                Hasta hace poco tiempo,  los restos fósiles más antiguos que se conocían correspondían a estructuras estromatolíticas halladas en rocas pertenecientes al cratón de Pilbara, en la región de Warrawoona (Australia), con una edad de 3.500 millones de años, lo que los sitúa dentro del eón Arcaico (4.000 m.a – 2.500 m.a).

                Sin embargo, hace un par de meses, apareció la noticia de que en  el cinturón supracortical de  Isua (suroeste de Groenlandia)  un equipo liderado por el Prof. Allen Nutman, de  la Universidad de Wollongong (Australia) descubrió lo que considera son los estromatolitos fósiles más antiguos que han sido hasta ahora datados, con una edad de 3.700 millones de años; serían las primeras estructuras visibles   halladas en las rocas de Isua.

Los investigadores  Allen Nutman y Vickie Bennet muestran la roca sedimentaria que contiene  el fósil de 3.700 millones de años. ( http://www.hispantv.com)
 

                De acuerdo al nuevo estudio, que fuera publicado en la revista Nature, los fósiles recientemente descubiertos superan en 200 millones de años a los  estromatolitos de Warrawoona,  que hasta ahora constituían la evidencia más antigua de vida en nuestro planeta. 

                Isua es considerada la cuna de la vida, ya que en ella se encontraron las rocas de más edad que se conocen y que presentan un buen estado de conservación.  Los fósiles hallados ahora, salieron a la luz  después del deshielo de una capa de nieve que los cubría.  Para los autores de la investigación, “encontrar pruebas de vida parecía imposible en estas rocas deformadas y alteradas”, haciendo al referencia al hecho de se trata del tipo metamórfico, las cuales  son sometidas a deformaciones por el calor y la presión, con el paso del tiempo, lo que hace casi imposible la preservación de material de origen orgánico.   Pero, para sorpresa de los científicos, un área pequeña  permaneció intacta,preservando  sus características sedimentarias; fue allí donde hallaron los estromatolitos, cuya altura va entre  1 a 4 cm.

Representación de la Tierradurante el Hadeico. (http://geologicalman.blogspot.es/tags/hadico/)
 


                Los mismos son fósiles comunes en todas las eras geológicas y en diversos entornos,comprendidos desde ambientes marinos poco profundos hasta fuentes hidrotermales. Aún así, no suele ser fácil encontrarlos.  Son estructuras órgano sedimentarias y laminadas, que crecen adheridas al sustrato;  se forman capa por capa, por la acción de cianobacterias, que facilitan  la precipitación de  carbonatos.

                 Para el profesor Nutman, este nuevo descubrimiento implicaría que la vida debió haberse originado mucho antes de lo pensado;posiblemente a finales del eón Hadeico (4,65 mil millones de años – 4000 millones de años), cuando la Tierra  se formó a partir de materia que se encontraba en órbita alrededor del sol. Esto dejaría un pequeño intervalo de tiempo para la evolución de la vida, situando este proceso en una época en la que nuestro planeta estuvo sometido al constante bombardeo de asteroides.

                Además,para el investigador,   los estromatolitos de Isua indicarían que hace unos 3.700 m.a ya existía más diversidad de la que pensábamos, demostrándose así la antigüedad del código genético.        

                Gracias a este hallazgo,  podría capturarse al menos una pequeña parte de la historia temprana de nuestro planeta y probar que la vida  emergió de manera rápida durante épocas tempranas.

                Cabe mencionar, además, que científicos de la Universidad de Wollongong ya habían desarrollado varias  líneas de investigación, encontrando  posibles evidencias de organismos vivos de hace aproximadamente 4000 millones de años.

Representación de la superficie de la Tierra, 3.000 m.a., durante el eón Arcaico. (http://www.arcadiastreet.com/cgvistas/earth/01_precambrian/earth_01_precambrian_2200.htm)

                Si bien han surgido detractores, quienes alegan que es difícil determinar las características biológicas de los estromatolitos hallados en Groenlandia, y que en realidad se trata  de rocas, el equipo de Wallangong está más que convencido de que puede confirmar el origen orgánico de los mismos y que el tiempo les acabará dando la razón acerca de que se trata de los restos de seres más antiguos que han habitado la Tierra, conocidos hasta la fecha.

                Tocará esperar a que se produzcan más  hallazgos que nos permitan llegar hasta el primer minuto de vida del primer organismo que surgió, a partir del cual la vida se abrió camino en aquel entonces joven mundo.

Por Paula Aguilera

Fuentes

  • Allen P. Nutman, Vickie C. Bennett, Clark R. L. Friend, Martin J. Van Kranendonk, Allan R. Chivas. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structuresNature, 2016; DOI:10.1038/nature19355
  • University of Wollongong. (31 de agosto de 2016). Life thrived on young Earth: scientists discover 3.7-billion-year-old fossils: Remarkable find by team of Australian researchers points to earliest existence of diverse life on Earth. ScienceDaily. Recuperado en fecha 14 de noviembre de  2016 de www.sciencedaily.com/releases/2016/08/160831172441.htm
  • Achenbach, Joel. 3,7 billion year old fossils may be the oldest  signs of life on Earth. The Washington Post (en línea) 31 de agosto de 2016. (fecha de consulta: 14 de noviembre de 2016) Disponible en: https://www.washingtonpost.com/news/speaking-of-science/wp/2016/08/31/3-7-billion-year-old-fossils-may-be-the-oldest-signs-of-life-on-earth/
  • Hoy Los Ángeles.  Fósiles parecidos a conos antiguos sugieren que la vida en la Tierra podría haber iniciado 3,700 millones de años.  Diario Hoy Los Ángeles (en línea) 1 de septiembre de 2016. (Fecha de consulta: 15 de noviembre de 2016). Disponible en: http://www.hoylosangeles.com/latimesespanol/hoyla-fosiles-parecidos-a-conos-antiguos-sugieren-que-la-vida-en-la-tierra-podria-haber-iniciado-hace-3-7-20160901-story.html

Paula Aguilera

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