Some species can be defined as “mosaic.” In other words, that they possess a combination of features from two other species. These species are particularly useful, revealing evolutionary relationships between organisms. Watching traits present in a later species emerge in an earlier creature, changing the average early creature to become more and more like the later form is a good indication there’s an evolutionary relationship being observed.
But that’s not the limit of their usefulness. They can help show which of the later species’ traits first emerged, how they changed and so forth. For example, it was thought that various changes to the human hip were the product of adapting to large brained babies. Yet Australopithecus sediba had many of these hip adaptations despite the fact its brain wasn’t much bigger than a chimps.
Mosaic finds are particularly prevalent in the human fossil record, with several species appearing to be “partway” between two others. Archaic Homo sapiens, for example, are intermediaries between us and Homo erectus. The aforementioned Australopithecus sediba is a mosaic of Australopithecus africanus and later Homo (such as Homo erectus).
As you may begin to realise, these mosaic forms are not only useful scientific discoveries but sterling evidence for evolution as well. They are the “transitional forms” creationists are always demanding yet claiming are never found. Unfortunately, the mosaic nature of these fossils leaves creationists with an out. Since they do contain a mixture of two species the scientific descriptions go “look how feature X looks like species X and look how feature Y looks like species Y.” A creationist intent on obscuring the transitional nature of these fossils could only focus on the first part of the sentence, going “this isn’t a transitional form, it’s just a member of species X!”
For an example, let us return to Australopithecus sediba. The paper which first described the find is “Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa” and notes
The closest morphological comparison for Au. sediba is Au. africanus, as these taxa share numerous similarities in the cranial vault, facial skeleton, mandible, and teeth. Nevertheless, Au. sediba can be readily differentiated from Au. africanus on both craniodental and postcranial evidence….these exact differences also align Au. sediba with the genus Homo
And which part of this description does Answers in Genesis, a multi-million dollar creationist organisation, quote when they attempt to dispute the fact Au. sediba belong in the human family? Well here’s an extract from their article “The Problem with Australopithecus sediba“
The Berger team also write: “The closest morphological comparison for Au. sediba is Au. africanus.” My own cast of an Australopithecus africanus skull confirms its likeness to Australopithecus sediba, and pictures of such skulls confirm the same. And Australopithecus africanus also closely resembles a modern chimpanzee
Au. sediba is just Au. africanus, and since Au. africanus is just a chimp we can ignore the whole thing! Including the bits that describe how Au. sediba differs from Au. africanus, rendering our chain of reasoning invalid (even assuming Au. africanus was just a chimp).
But surely this is just a one off! Well, in “The Truth about Human Origins“, a book from ‘Apologetics Press’ the authors discusses Ardipithecus ramidus.
The authors of the paper in Nature described the cranial fossils as “strikingly chimpanzee-like in morphology”. The pieces of arm bone were described as exhibiting “a host of characters usually associated with modern apes”.
The paper itself, from when the species was still called Australopithecus ramidus, actually said
The specimen also shows a host of characters usually associated with modern apes, including a strong angulation of the distal of the distal radial articular surface due to a large styloid process, a strong lateral trochlear ridge on the distal humerus (also seen in some A. afarensis), and an elngate superoposteriorly extended lateral humeral epicondyle. The Aramis arm diverges from the African ape condition in other features…
As if the second, omitted sentence isn’t bad enough note the few words from before their chosen quote they ignored. “The specimen also shows.” Sure enough, just before this paragraph the article noted various non-ape like characteristics.
The arm displays a mosaic of characters usually attributed to hominids and/or great apes. From proximal to distal, probable derived characteristics shared with other hominids include an elliptical humeral head; a blunt proximally extended ulnar olecranon process surmounting a straight dorsal upper shaft profile; an anteriorly orientated trochlea notch; and an anteriorly facing ulnar brachialis insertion. The specimen also shows a host of characters usually associated with modern apes
Immediately proceeding and following the chosen quote are descriptions of non-ape characteristics completely omitted so as to make Ar. ramidus seem more like an ape than it actually is. This lie of omissiom forms a crucial point later on, with them arguing
A[ustralopithecus]. anamensis is “more or less” a hominid, even though it is similar to Ardipithecus ramidus (which, as Donald Johanson admitted, possesses “many chimp-like features”).
Now, my point here isn’t to try and poison the well or guilt by association to persuade you to dismiss all creationist quotes or arguments as unfounded. I’m simply trying to point out that mosaic or “transitional” forms are, by their very nature, open to being misquoted to lend unfounded credence to a position.
They’re not arguing against the non-ape like features in these hominins or presenting other valid reasons why they should be considered apes, they’re just completely ignoring the issue. Whether you’re accept creationism or evolution, hopefully you can appreciate that is wrong. So I urge you, whether you’re a creationist or not, when presented with an article on how a mosaic find is “just an x” – especially if a quote is used to lend credence to this idea – be skeptical. Don’t dismiss it but investigated it if possible. At the very least take it with a mountain of salt.