There are two ways to account for the origin of life on this planet: spawning or seeding. According to Spawning Story, the geochemistry of the ancient Earth spawned some form of self-replicating system which then evolved into proto-cells which then evolved into modern-day cells. According to the Seeding Story, the original cells were deposited on the ancient Earth from beyond the confines of the ancient Earth.
If the original cells were intentionally used to seed this planet by some form of intelligence billions of years ago, how could we ever hope to detect such an event?
In 1973, Francis Crick and Leslie Orgel suggested two lines of biological evidence that would support directed panspermy, one version of the Seeding Story . First, they suggested “the presence in living organisms of elements that are extremely rare on the Earth might indicate that life is extraterrestrial in origin.” While such evidence would speak to panspermy, it really doesn’t indicate an intelligent origin of such life forms. On the contrary, if one is going to intentionally seed the ancient Earth, it would not make much sense to design the first cells such that they were dependent on something the Earth could not provide. An intelligent solution would seek to maximize the chance of a successful seeding, thus design the first cells such that the Earth would be able to sufficiently supply their raw materials. Crick and Orgel originally suggested the element molybdenum might be a candidate indicating an extraterrestrial origin, noting both its importance to life and extreme rarity on Earth. However, several decades later, Simon Conway Morris informs us that their estimates were much too pessimistic, as the Earth appears to contain sufficient amounts of molybdenum .
Their second line of biological evidence has greater potential. They pointed to the genetic code and noted:
It is a little surprising that organisms with somewhat different codes do not coexist. The universality of the code follows naturally from an “infective” theory of the origins of life. Life on Earth would represent a clone derived from a single extraterrestrial organism. 
Thus, the Seeding Story not only entails some form of discontinuity (historical or chemical) between life and non-life, but it also predicts some form of universality among the life forms as a consequence the first cells functioning as stem cells.
How much universality does the Seeding Story actually predict? This is difficult to say. On one hand, we would expect a significant amount of universality not merely from the fact of the original cells functioning as stem cells, passing on their endowed features and components to their evolutionary descendents, but also because we might expect certain design principles and strategies to be employed in different contexts. On the other hand, to maximize the chances that the seeding would be successful, we would not want to put all our bacterial spores into one basket. It would be wiser to seed with a consortium of different types of bacteria, thus increasing the odds that at least one would take root in the unpredictable environment of the ancient Earth. But the choice to use a diverse set of microorganisms goes deeper than this. In the 1950s, ecologists recognized that there was a positive relationship between the diversity of a biological community and its stability . Paul Ehrlich, a controversial environmental biologist from Stanford University, explains the relationship as follows:
Complex communities, such as the deciduous forests that cover much of the Eastern United States, persists year after year if man does not interfere with them….A cornfield, which is a man-made stand of a single kind of grass, has little natural stability and is subject to almost instant ruin if it is not constantly managed by man. 
Biologist Robert MacArthur noted that “stability increases as the number of links increases” as such stability is easier to achieve with a more diverse assemblage of species . Writing in Nature, Kevin Shear McCann summarizes the most current thinking along these lines:
Taken together, recent advances indicate that diversity can be expected, on average, to give rise to ecosystem stability. The evidence also indicates that diversity is not the driver of this relationship; rather, ecosystem stability depends on the ability for communities to contain species, or functional groups, that are capable of differential response . 
Thus, by seeding the planet with a diverse set of microorganisms, the seeding event is front-loaded to more quickly establish an ecosystem or a network of symbiotic links between different organisms to set up a community that is capable of differential response in the face of environmental stresses. Yet this very diversity steers us back to core universality. If the various cells among the original consortium were all built from completely different building blocks (amino acids, nucleotides, sugars, etc.), requiring radically different environments to satisfy their radically different needs, the number of links between individual populations would be greatly diminished. In any symbiotic relationship, the two parties must share enough in common so at least one party may benefit. For example, predators can consume prey only because their prey is built from the same building blocks. Organisms existing in a mutualistic relationship can trade with each other only because they too are built from the same material. Thus, if the original seeding event was due to intelligent intervention, we might expect to find a pattern of biological diversity laid on top of a deeper universality. The diversity of biological features would echo the intent to enhance the chance for a successful seeding, while the deeper universality of biological features would follow from the original cells functioning as deposited stem cells (Crick and Orgel’s argument), exhibiting common design strategies to similar problems, while also reflecting an attempt to maximize the success of seeding by allowing the different cellls to connect with each other.
1. Crick, FHC and Orgel, LE. 1973. Directed panspermia. Icarus 19: 341-346.
2. Conway Morris, S. 2003. Life’s Solution: Inevitable Humans in a Lonely Universe. Cambridge University Press; Cambridge, p. 26.
3. Elton, C.S. 1958. Ecology of Invasions by Animals and Plants. Chapman & Hall, London.
4. As cited in Barrow, JD and Tipler, FJ. 1988. The Anthropic Cosmological Principle. Oxford University Press; Oxford, p. 141.
5. MacArthur, R.H. 1955. Fluctuations of animal populations and a measure of community stability. Ecology 36:533-536.
6. McCann, KS. 2000. The diversity-stability debate. Nature 405: 228-233.