Scientists have confirmed for the first time that an important component of early genetic material which has been found in meteorite fragments is extraterrestrial in origin, in a paper published on 15 June 2008.

The finding suggests that parts of the raw materials to make the first molecules of DNA and RNA may have come from the stars.

The scientists, from Europe and the USA, say that their research, published in the journal Earth and Planetary Science Letters, provides evidence that life’s raw materials came from sources beyond the Earth.

The materials they have found include the molecules uracil and xanthine, which are precursors to the molecules that make up DNA and RNA, and are known as nucleobases.

The team discovered the molecules in rock fragments of the Murchison meteorite, which crashed in Australia in 1969.

They tested the meteorite material to determine whether the molecules came from the solar system or were a result of contamination when the meteorite landed on Earth.

The analysis shows that the nucleobases contain a heavy form of carbon which could only have been formed in space. Materials formed on Earth consist of a lighter variety of carbon.

Lead author Dr Zita Martins, of the Department of Earth Science and Engineering at Imperial College London, says that the research may provide another piece of evidence explaining the evolution of early life. She says:

“We believe early life may have adopted nucleobases from meteoritic fragments for use in genetic coding which enabled them to pass on their successful features to subsequent generations.”

Between 3.8 to 4.5 billion years ago large numbers of rocks similar to the Murchison meteorite rained down on Earth at the time when primitive life was forming. The heavy bombardment would have dropped large amounts of meteorite material to the surface on planets like Earth and Mars.

Co-author Professor Mark Sephton, also of Imperial’s Department of Earth Science and Engineering, believes this research is an important step in understanding how early life might have evolved. He added:

“Because meteorites represent left over materials from the formation of the solar system, the key components for life — including nucleobases — could be widespread in the cosmos. As more and more of life’s raw materials are discovered in objects from space, the possibility of life springing forth wherever the right chemistry is present becomes more likely.”

Source: Imperial College London

“Extraterrestrial nucleobases in the Murchison meteorite”, Earth and Planetary Science Letters, Sunday 15 June 2008.

Josh says:

Since we know that nucleobases form well in ice, it seems logical for these molecules to be in meteors. In the early Earth, there were a lot of meteors hitting, so these nucleobases were probably the most common, relatively complex molecules available. It seems natural then that since they can so easily polymerize, that they did, forming the initial enzymes (proteins evolved later. RNA can act as an enzyme, and in fact still does in our bodies now. Take for example the splicing out of introns).

When the first cells developed, how could they bring molecules from the environment into their living interior without the specialized structures found on the modern cell membrane? A research team from Massachusetts General Hospital (MGH) has found that the sort of very simple membrane that may have been present on primitive cells can easily allow small molecules – including the building blocks of RNA and DNA – to pass thorough. Their report will appear in the journal Nature and is receiving early online release.

“We have found that membranes made from fatty acids and related molecules – the most likely components of primitive cell membranes – have properties very different from those of the modern cell membrane, which uses specialized pumps, channels or pores to control what gets in and out,” says Jack Szostak, PhD, of the MGH Department of Molecular Biology and Center for Computational and Integrative Biology, the report’s senior author. “Our report shows that very primitive cells may have absorbed nutrients from their environment, rather than having to manufacture needed materials internally, which supports one of two competing theories about fundamental properties of these cells.”

How nutrients could get into cells without the specialized mechanisms of the modern cell membrane has been a mystery. The environment in which primitive cells formed probably included many types of fatty acids, which could have been supplied through a couple of scenarios. Fatty-acid molecules could have been formed by the action of heat and minerals deep beneath the earth’s surface and then brought to the surface through deep-sea vents, hot springs or geysers; or they could have come to the earth’s surface on meteorites. No matter the original source, when fatty acids are concentrated in water, they will naturally assemble into membranes which then close into tiny spherical structures called vesicles.

Szostak’s team carefully analyzed vesicles comprised of different fatty acid molecules and identified particular features that made membranes more or less permeable to potential nutrient molecules. They found that, while large molecules such as strands of DNA or RNA could not pass through fatty acid membranes, the simple sugar molecules and individual nucleotides that make up larger nucleic acids easily crossed the membrane.

To further explore the function of a fatty acid cell membrane, the researchers used activated nucleotides they developed for this study that will copy a DNA template strand without needing the polymerase enzyme usually required for DNA replication. After placing template molecules inside fatty-acid vesicles and adding the activated nucleotides to the external environment, they found that additional DNA was formed within the vesicles, confirming that the nucleotide molecules were passing through the fatty-acid membranes.

“Today we have complex cells living in a chemically simple environment, but the primitive environment was chemically very complex, allowing for the synthesis of complex organic chemicals that cannot be formed in today’s environment,” Szostak explains. “We think that the first cells were very simple and assembled from molecules present in localized environments on the early earth.”

For many years, Szostak’s team has been working on the development of a ‘protocell’ that would replicate probable features of the earth’s first cells. “The chemistry of nucleic acid replication is the remaining hard part,” he says. “We’re putting a lot of effort into nucleic acid chemistry, but there are also other interesting and important questions – like how cells made the transition from very leaky early membranes to today’s very impermeable membranes – that we are starting to study.”

Source: Massachusetts General Hospital

Template-directed synthesis of a genetic polymer in a model protocell. Sheref S. Mansy, Jason P. Schrum, Mathangi Krishnamurthy, Sylvia Tobé, Douglas A. Treco & Jack W. Szostak. Nature. doi:10.1038/nature07018

Josh says:

This reminds me about when I read that Stanley Miller, who demonstrated that organic molecules could have formed from inorganic molecules such as ammonia and cyanide in an early Earth environment. He recently found that when water with cyanide and ammonia is frozen, RNA molecules form and polymerize into chains over a short period of time (25 years).