It is intriguing to consider the consequence that shaking hands with an alien could have, whether this was from a literal, E.T.-esque encounter, or perhaps more plausibly, from a metaphorical ‘handshake’, for example if we discovered alien microbes on mars and studied them in the laboritory back on earth. It is even more intriguing to imagine what such encounters could teach us about the fundamental principles of life. However could they also pose a risk to life on earth? I am not referring to a War of the Worlds style alien invasion, but for example could an extra terrestrial virus, inadvertently released, go on to parasitize our cells? Could such a life form even swap genes with our planet’s organisms, dramatically altering the course of evolution on earth? These possibilities may be rather far fetched, but perhaps they are not entirely implausible.
The earth is home to an incredible diversity of life, perhaps 100 million species, broadly divisible into three domains; the Eukaryotes (everything from amoeba to blue whales), Bacteria, and Archaea (superficially similar to Bacteria but perhaps evolutionarily closer to Eukaryotes). In a sense, however, the earth only possesses one form of life. That is, all species seem to descend from a single common ancestor, and all share the same basic molecular machinery.
Molecular basis of earth’s life
A simple description is that cells build themselves using proteins, and store the information for each protein as a gene: a section of DNA. RNA acts as an intermediate messenger between the two, and is a similar molecule to DNA; a gene is first copied into messenger RNA which acts as a template for the assembly of a specific protein. DNA and RNA store information in the form of just four types of bases, which can be thought of as letters in a language. A specific sequence of three such bases (a codon) specifies a particular amino acid (the building block of proteins). Amazingly each codon ‘word’ specifies the same amino acid in every living thing (with a couple of minor variations); in molecular terms we all speak the same language. It is this that allows the technology of genetic modification to work, for example a bacterial gene for an insecticidal protein has been put into crops, and in its new host the gene can be read and so produce its protein product.
Left: from DNA to protein - life's molecular basis, Right: life's language - translating the codons of RNA into the amino acids of protein.
It is interesting that although similar to DNA, RNA also has the ability to act as part of the cell’s machinery, much like a protein. For example the machines that assemble proteins are the ribosomes, and are largely composed of RNA. The finding that RNA can act both in storing information and as a molecular machine, has led some scientists to propose an RNA world hypothesis, in which the first life had only RNA, and lacked DNA and proteins.
Darwin proposed that life could be described using an evolutionary tree, much like a family tree. More recently, techniques such as genome sequencing have allowed us to better understand life’s evolution, and the picture that has emerged actually turns out to be more of a web than a tree. When DNA from one species has passed into another distantly related one, this has not always been the end of its existence.
So for example, within eukaryotic DNA there are parasitic elements named transposons. They serve no clear purpose but repeatedly copy themselves within our DNA, and they sometimes disrupt other genes. Transposons may originate from viruses no longer able to package themselves and infect new hosts, or conversely some viruses could have originally evolved from transposons. It has recently been shown that transposons are so good at spreading themselves that one has hopped from a human into the gonorrhea bacterium, which invade human cells. Gene transfer has also been found to occur in the other direction: from a bacterium into its Eukaryotic host. Bacteria called Wolbachia infect insect cells, and in one case almost all Wolbachia’s genes have transferred into a fruitfly species. In this case some of the genes are even active in the new fruitfly host.
Following transfer of a gene between two species, it may no longer serve a purpose and eventually be lost from its host, or may survive as a parasite, as in the case of transposons, but these are not its only possible fates. The gene’s new host may find that the protein it produces is useful, and so the host retains the gene. Such lateral transfer of useful genes are common amongst bacteria. During conjugation, one bacterium attaches to another and transmits a small loop of DNA (a plasmid). The plasmid encodes the proteins necessary for conjugation, and sometimes proteins beneficial to the host for example proteins for metabolising specific chemicals.
An example closer to home are the mitochondria; structures in our cells which generate most of our chemical energy. These appear to have originated from a bacterium which was taken up into a eukaryotic cell, and subsequently lived symbiotically within it (to the benefit of the bacteria and its host). Mitochondria still contain their own DNA in a form close to that of bacteria (they have a loop of DNA, containing genes related to those of alpha-proteobacteria). Interestingly over time many mitochondrial genes appear to have transferred to the chromosomal DNA in the cell’s nucleus, where most of the cell’s genes are stored. A very similar situation appears to have happened in the case of chloroplasts; the structures in plants and algae where photosynthesis occurs. In this case the origin is the photosynthetic cyanobacteria.
Viruses also undergo frequent genetic transfer with other species. They lack many of the molecular components shared by the three domains of life, generally consisting of just a small amount of DNA or RNA encapsulated in a few types of protein, and they cannot replicate in isolation. However the fact that they use the same basic DNA/RNA code, allows them to hijack the machinery of other cells to replicate.
Some, like HIV, can copy themselves into their host’s DNA as part of their replication process. Occasionally they become permanently integrated and passed down the generations. Although this is the cause of several inherited diseases, during pregnancy we actually seem to use viral genes gained millions of years ago, in the mechanisms protecting the embryo from its mother’s immune system. Pregnancy is a fascinating challenge faced by placental mammals, as the embryo is genetically different from the mother, and we may expect it would be attacked by the mother’s immune system. It is known that retrovirus like proteins are expressed in the placenta, and they play an important role in fusing cells together into a barrier called the syncytia. This lies at the interface between the placenta and the mother’s cells, allowing through nutrients and oxygen, whilst acting to block the mother’s immune cells. This ability of the viral protein to fuse cells together is seen for some types of pathogenic viruses which use it to spread to neighbouring cells.
So exchanges of genes between species are widespread on earth. Could such a transfer occur between us and an alien species? For this the alien would need to speak the same language as us in molecular terms (ie. be formed from proteins coded for by DNA/RNA) . Of course the likelihood that precisely the same form of life arose separately on two planets is nearly zero. There are however several proposals which could mean we share a common ancestor. One is panspermia: the hypothesis that life on earth began elsewhere. For example bacteria such as Deinococcus radiodurans, which can withstand extreme cold and high radiation levels, could have hitched a ride here on a comet.
Even more bizarre is directed panspermia proposed by Francis Crick (of Watson and Crick fame) and Leslie Orgel. This proposal is that intelligent aliens could have sent out space craft to seed simple life forms on new planets. They argued that even within our galaxy, planets are likely to have formed long before our own solar system came into existence. Perhaps an intelligent life form evolved on one of these. They may have been motivated by discovering or believing that other planets they observed in space were habitable, but as yet uninhabited, and so devised a programme to seed life on them. There may be a problem with such a motive, which could also argue against humans undertaking such a programme, and that is the possibility of the alien life wiping out the existing life or even advanced societies on the target planet. Even if it could be demonstrated that life did not exist on the target planet, life could emerge on it during the long flight times of such a mission. Of course the aliens could have been motivated by all kinds of other reasons. More recently astronomers have been discovering huge numbers of planets beyond our solar system, including planets such as the ‘earth like’ Kepler 22-b where liquid water is likely to exist. It is therefore looking increasingly likely that life with a similar molecular basis to that on earth could be sustained elsewhere in space.
If, by whatever means, earth life has extraterrestrial origins, then perhaps we could share a common molecular language with aliens, and the transfer of functioning genes with them would not be impossible. Caution is therefore advised, before shaking hands with aliens.