Transplanting organs from animals to humans Chimp hearts have been successfully transplanted into humans... pig hearts too for that matter. Look up xenotransplantation. Evidence in science that connect the animals and humans Comparative anatomy, genetics, the biogeography, fossil record. The latter provides a series of links, from apes (keep in mind, we didn't just evolve from apes, we still are apes) to well before. http://en.wikipedia.org/wiki/Timeline_of_human_evolution" [Broken] ----- most of the classification categories refer to common ancestors (and traits) we have with other animals: Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Primates Family: Hominidae Subfamily: Homininae Tribe: Hominini Genus: Homo Species: H. sapiens There's yet more subdivisions, eg sub-phylum Vertebrata. There's also better and more recent evidence, largely called http://en.wikipedia.org/wiki/Phylogenetics" [Broken]. Justification for the missing link What are popularly referred to as "missing links" are just snapshots of more obviously distinguishable "generations" along the evolutionary path. There are no clear demarcations between species or genus or family or so on and so forth. If you could magically line up and look at EVERY reproductive generation between, lets look on the "short" scale, modern Homo sapiens and our last common ancestor with Pan, you would see EVERY generation give birth to its own species, only seeing notable differences when you jumped ahead many, many generations, in which case you'd simply be seeing the cumulative effect of countless tiny generational changes piling up on one another (rather than any one big change or complex of multiple changes occuring simultaneously to create an offspring taxonomically distinct from the parent; this does NOT happen.) As for multiple species evolving from a common ancestor, heres this for a simple comparison; a modern human family spreads out and splits, acquiring subtle changes in customs or mannerisms or surname, so on and so forth. Several generations down the line the original "family" is now a number of distinct, independent families not obviously related to one other. The family may share common traits, but they've developed along various lines, and none likely perfectly resemble their common ancestors in any of these traits, though some more traditional lines of descent may more closely resemble the ancestors they all share. (A family farming out in rural china more closely resemble ancestors of the family as they were hundred years ago, than that family's cousins who moved to the US and became Americanized.) The same general thing happens to organisms. Some populations diverge into multiple lines of descent, slowly acquiring changes, some remaining more traditional, others becoming more obviously distinct, while the original ancestors are dead, as ancestors tend to be, no matter how closely this or that surviving population may or may not resemble them.