I would asked them for some evidence. Questions like, how is our
biochemistry constructed. What are the chemicals in our genetic inheritance,
how is the genetic molecule (DNA but I wouldn't tell them that) transformed
into the working molecules (proteins, but I wouldn't tell them that either.
I would also check to see that the star or galaxy that the signal came from
is more than 75 light-years distant. If it is not, then they could have
picked up information from our TV and radio signals.
I would ask the means by which the seeds were delivered here and check the
feasibility of that. Afterall, it might be an alien race with a God complex.
Passing those tests, and maybe some others I could think of, I think many
evolutionists (not me) would consider their claim. As far as I am aware,
panspermia, the seeding of life on earth from other worlds, was first
proposed in 1903 by S. Arrhenius in Lehrbuch der Kosmischen Physik. He said
that small particles could carry life from one solar system to another.
Fred Hoyle and Chandra Wickramasinghe have also advocated the transfer of
life from one world to another via cosmic bodies. (See the two books Living
Commets and Diseases from Space as outgrowths of this view).
Thus, I think many evolutionists would be willing to consider the claim of
design of life from another world.
Foundation, Fall and Flood