If life on Earth has extraterrestrial origins, would it be possible to find hints of it in our DNA?

Some believe that life on Earth was transmitted from an icy comet nucleus. If extraterrestrial DNA material was preserved by an icy comet that hit the Earth and seeded our planet with life, might it be possible to find signatures of that extraterrestrial lifeform in the DNA of all life on Earth? Has there been efforts to trace life as far back as possible, comparing the DNA of life? Also, I remember learning that there are actually more than four amino acids, but our DNA only uses four. Could an extraterrestrial lifeform's DNA have used more than four amino acids, but only four amino acids survived the freeze and long journey in the hostile environment of space of its host comet?