SCIENCE

Bit of old Earth

Ancient rocks hints at Earth's origin

 

     The rocks collected on Baffin Island were formed during a volcanic                                                                                                        eruption 60 million years ago. Obaidur Rahman

 

Recently geochemists studying Arctic rocks have discovered evidence of ancient rock from the Earth's interior, which, they claim, is as old as the Earth itself. Much of the material that made up the early Earth was lost and modified as the processes of melted mantle began firing up magma to form Earth's crust and plate tectonics then mixed that crust back into the mantle below. Team-leader Dr. Matthew Jackson, Geochemist, Boston University and his group of international researchers discovered this recently from the lava samples of Greenland and Baffin Island in the Canadian Arctic only suggest this astounding possibility. Beneath this particular environmental setting lies a region of the Earth's mantle that has largely escaped the billions of years of melting and geological shake-up that has affected the rest of the planet. It must be understood that the mantle is the layer of viscous, super-hot rock that stirs below the solid crust of the planet's surface. Naturally, when covered by 5-to-25-mile-thick crust, this mantle, which actually makes up four-fifths of Earth's volume, is difficult to get to and study. And even when samples of the mantle are found, they usually happen to be relatively young and recycled, emerging at the surface of Earth as molten volcanic material. But the rocks that Dr. Jackson & company discovered were spewed up to the surface of Earth by an eruption about 60 million years ago. Scientists believe that the newly discovered mantle “reservoir” as it is now called, dates from just few tens of millions years after the Earth was first assembled from the collisions of smaller bodies. It is also believed that this reservoir very likely houses the basic elements of compositions of the mantle shortly after the formation of the Earth's interior core. The same very elements that existed before the 4.5 billion-years' process of crust formation and recycling, modified the composition of most of the rest of the Earth's interior. According to co-author Dr. Richard Carlson of Carnegie Institution's dept. of Terrestrial Magnetism, “This was a key phase in the evolution of Earth. It set the stage for everything that came after”.
The team of scientists examined a variety of isotopes to asses the exact age of the rocks. It must be mentioned here that elements can occur in various forms which are known as isotopes and over-time ratios of these isotopes gradually change and this allows the researcher to estimate the rock's age. Studies of helium isotopes have shown that rocks have anomalously high ratios of helium-3 to helium-4. This helium-3 is generally extremely rare within the Earth whereas helium-4 is constantly replenished within the Earth by the decay of radioactive uranium and thorium. This high proportion of helium-3 strongly suggests that the lavas in Baffin Island must have come from a reservoir in the mantle that had never previously out-gassed its original helium-3. And this also strongly implies to the scientists that neither had it been subjected to the extensive chemical differentiation experienced by most of the mantle. Also by analyzing the lead isotopes in the lava samples, the researchers were able to date the rocks to between 4.55 and 4.45 billion years, slightly younger than the Earth, herself!
Researchers are hoping that this significant finding will offer a more detailed insight into Earth's early geo-chemical evolution, all the dynamics that are at play in the Earth's interior as well as even suggesting the possibility that this finding could even challenge the existing theories regarding the formation of Earth.

Origin of modern humans

According to our knowledge of evolution, life has originated 3.5 billion years ago in a form of single cell wrapping essential molecules required to form life. After that, complexity came in life forms, multicellular organisms emerged, and evolution turned to macroevolution from microevolution. The first dinosaurs appeared 245 million years ago and dominated the earth till 65 million years ago. A comet or sudden meteoric shower on earth was, probably, responsible for their extinction. Ancient humans originated later after that. I am going to focus the origin of modern humans according to paleontology, the earth science that studies fossil organisms and related remains, and genetic evidence found to date. Since the separation of archaic humans, the hominids, from the apes around 5-7 million years ago the evolutionary tree generated branches for the newer ones. From the hominids, the genus homo had diverged around 2 million years ago in Africa.
It is hypothesized through paleoanthropological study and genetic evidence that around fifty or seventy years ago, a group of few hundred or even several thousand Africans left Africa forever. The reason they left their motherland might be climate change or scarcity of food supply from the ocean although the reasons are not clear. However, they started a journey, a journey to remember, a journey of human history taking with them the physical and behavioral traits, the intelligence including capacity for language which characterizes modern humans. Scientists are getting the insight of this historical journey by doing research on fossilized bones or spearheads. However, modern technology and the advent of molecular biology have offered more sophisticated ways to discover the insight of that journey. Although the paleoanthropologists are more focused on fossils rather than the DNA, the genetic information could be a better option to find out the markers present there to make the evolutionary tree.
There are, in fact, two theories regarding the origin of modern humans, one is out-of-Africa hypothesis that postulates that humans with modern traits left Africa from fifty to seventy thousand years ago to settle the world. In doing so, they replaced the archaic hominids, such as Homo erectus that left Africa as early as 1.8 million years ago. This out-of-Africa hypothesis gained support after the discovery of so called “Mitochondrial Eve” in 1987, when Rebecca L. Cann and Allan C. Wilson at the University of California, Berkeley, published a groundbreaking paper based on mitochondrial DNA analysis and concluded that humans from different population all descended from a single female in Africa who lived about 200,000 years ago. This out-of-Africa hypothesis is also called single origin theory or total replacement theory due to its nature of little or no interbreeding among the ancient human populations such as Homo Neanderthals and Homo erectus that left Africa. On the other hand, the multiregional theory or so called candelabra theory of regional continuity proposed by Milford H. Wolpoff in 1988 suggests that diversity from within Homo erectus and has confined over several million years, with some exchange of genes. It gives the idea that the modern characteristics of humans evolved not just in Africa but in archaic hominid population in Asia and Europe and interbreeding among all these groups ensured that they remained a single species. However, both the theory has proponents and opponents.
To get a clearer picture, therefore, more and more research both paleoantropological and genetic study is required. In fact, comparative genomics could be a very promising tool for the purpose because it may not be possible to get the identity of human population simply by analyzing a piece of DNA but comparative genomics may reveal the desired picture, may help to achieve the goal of the scientists working to unfold the mystery of the human evolution, the very origin of modern humans.

The writer is a Lecturer in Biotechnology, BRAC University.

What is cosmophobia?

 

 

Cosmophobia is an irrational fear that the world is about to end, and is sparked off by a belief among people that a cosmic end is near. The fear is ancient, and people have been readying for doom on and off - expecting floods, earthquakes, epidemics, drought, or even a collision with another planet. The latest bout was set off by the movie 2012, where the Mayan calendar counts December 21, 2012 as the last day. Earlier, the 2000 millennium frenzy had everyone believe that the end was near. For ages now, there have been apocalyptic predictions which have fuelled cosmophobia, but until now, they have all come to nought.