Ask an astrobiologist about their family tree and they might just hand you a 4.5 billion year old rock. Falling to Earth as meteorites, these fragments of our early Solar System may contain the first building blocks for life. If true, this would mean that the start of all life on Earth was not “on Earth” at all.
In the specks of dust that collided to construct the infant planets were elements that were radioactive. As their nuclei decayed to form more stable atoms, heat was released to warm the growing bodies of rock. For those forming in the cold reaches beyond Mars, these 1—100-km-size boulders were packed with ice. When their insides began to melt, hidden springs of liquid water appeared in which the first organic molecules of our Solar System began to be assembled.
There is little doubt this process happened. Meteorites that show signs of having once been exposed to water also host a feast of organic matter. Yet, the fact that these biological molecules could form in space does not mean that they ultimately began the life on Earth.
Life’s genetic code is blueprinted in the twisted strands of DNA and RNA which are made up of nucleobases. On the early Earth, nucleobases paired together to first form the simpler RNA, which then began the process of near-perfect replication. It was an evolutionary path that would one day lead to a human.
“To my mind, that is the start of life,” says Ben Pearce from McMaster University’s Origins Institute, lead author of a new paper that looks at the question of whether the seeds of life might have come from space. “To be able to create copies.”
To investigate whether our own nucleobases could have been delivered to Earth inside meteorites, Pearce started by examining the organic content found in meteorite falls.
Nucleobases come in five flavours: guanine (G), adenine (A), cytosine (C), thymine (T) and uracil (U). Pearce confirmed three of these were commonly listed in the meteoritic record, but there was no hint of cytosine or thymine. Intriguingly, these missing members are the complementary pairs to the discovered nucleobases, with guanine pairing with cytosine (G-C) and adenine pairing with thymine (A-T) in DNA.
“It is like everyone showed up to a ball, but they all forgot their dance partners,” co-author Ralph Pudritz commented.
This led to the question of whether these absences were just down to experimental bad luck, or if there was a reason why meteorites could not carry these counterparts of the genetic code.
To investigate this, the researchers turned to the chemical reactions that form these nucleobases. Exactly how successfully nucleobases are produced depends heavily on the construction materials available inside the early Solar System’s planetesimals. To find what would be around, the researchers looked to the comets.
Comets are icy rocks leftover from the planet formation process. Unlike many of the closer asteroids, comets have changed very little since their formation. This makes them an excellent snapshot of the conditions in the birthing pool of the first nucleobases.
Using comet composition as a starting point for their model, the scientists calculated the expected yields of the five nucleobases that make up our genetic code.
What they discovered was that while four of the nucleobases formed in measurable amounts, the elusive cytosine was missing. In truth, cytosine was created. However, it rapidly decayed within a few years to produce the more commonly found uracil nucleobase and ammonia. This meant the chances of finding cytosine in meteorite samples were practically zero.
"It's not that we simply haven't found any cytosine in meteorites,” Pearce exclaimed. “It seems that it can't be found!"
While this closed the case on the missing cytosine, the other absent nucleobase, thymine, seemed to be produced in detectable abundances. So why is it never seen?
It turns out that thymine decomposes in the presence of hydrogen peroxide; the same chemical in bleach and disinfectant. Hydrogen peroxide has been spotted in comets, making it a potential culprit for destroying any thymine that was formed.
These models explain the presence of only three nucleobases in the meteorite samples, but they leave a clear conundrum: if our genetic code needs five nucleobases, where did the missing two partners come from?
It is a question that still lacks a satisfactory answer. It is possible that these life seeds began on Earth, although this presents a number of tricky problems. Our early atmosphere was inhospitable for creating organic molecules, while our oceans run the risk of also producing only three nucleobases. A promising option is that the sun’s ultraviolet rays triggered the formation the organics on dust grains within our Solar System, which were then captured by the Earth’s pull.
“This is a big question!” Pearce concludes. “And at the moment, we don’t know the answer.”