What evidence is there for the missing link?

Answer 1

The idea of links, was derived from medieval theology's 'Great Chain of Being". The concept sported a parochial hierarchy of grater and lesser beings with mankind in dominion at the highest rank above animals, as the jewel of creation. Naturalistic ideas at this time were not spared from interjection with 'higher' metaphysical concepts, so mankind, was relegated to subservience below angels and archangels and so on, all the way up to God.

In some ways this 'chain of being' reflected upon (or from) the dominant social hierarchies, so prevalent in that time. The common classes, the aristocracy and royalty all had their place, and it was just so, because it was 'the done thing'. Anathema to this mindset, was the idea that one kind of being could ever become another, as if a servant, could move into the ruling classes. 'Good Lord... What ever will they think of next?'

By the 1860's the science of comparative anatomy, had begun to make inroads into the phenotypical relationships between modern primates (including humans - homo sapiens) the inferred kinship on the grounds of prima facie evidence was becoming hard to ignore. At that time there was no fossil evidence to support a tree of kinship in the modern hominid group.

By the early 1870's and with Darwin's "Decent of Man" the kinship of all living creatures had been preempted by more than a decade now by Darwin's "Origin of Species", so thinking people were pressed to consider the possibility that humans were a biological species (an animal) no different in principal than any other living creature. Darwin and Huxley found a receptive niche for the burgeoning idea of human/ape kinship. Darwin's idea of natural selection and kinship via common ancestors, had struck a chord. A chord that couldn't help resonating in the obvious biological structure of a particular hominid - homo sapiens. who after all, was made of flesh, bones, blood and similar organs and tissues as so many other commonly observable animals.

Nevertheless, the fertile idea seem to have spawned a few (equally fertile) misconceptions at about this time. One of those was the misconception that man sprung directly from apes. That is - modern humans evolved from modern apes. Even in educated circles, there were naturalists given to imagining something half way between modern humans and modern apes. The German evolutionist, Ernst Haeckel, went so far as naming the predicted 'half man half ape' using the conventional Linnaean system, 'pithecanthropus alalus' meaning, 'little bastard of the gap' (only kidding); it really means, 'ape man without speech'.

In latter years it was decided that it would be more prudent, to refrain from naming species before any artifacts were discovered, otherwise homo mermaidius might slip into the taxonomic charts. Good call on that one I say. Nevertheless, the hope of a mythical being can inspire and motivate people (look at god for instance), and by that token a Duch medical officer, Eugene Dubois took up the challenge to find our dearly beloved pithecanthropus. It was the dawning days of paleoanthropology and Dubois spent several years in the dedicated hunt. His reward finally came in the form of a hominid called 'Java Man' Pithecanthropus erectus. Although this hominid was actually an early species that resembles homo sapiens much more than the old world apes, the question of direct human ancestry was a moot point.

Since the time of Dubois, there has been a veritable avalanche of hominid fossils found, including nearly complete skeletons. In the case of australopthicus afarenses from the Afar region in Ethiopia we have a wealth of fossils including 'Lucy', and the 'first family' Many of these Australopithecien finds are thanks to the hard work of the Leaky family (lucky they weren't chosen to build the ark - it would have been the Leaky Ark - tee hee) and of course the ledgendary Donald Johanson and Tom Grey who discovered Lucy on the 24th of November, 1974.

The discovery of Lucy was the definitive cap on the 'missing link' controversy, as there were no doubts to Lucy's claim as a bi-pedal ancestor, and no question that she belonged to a different species either. Before this time several hominids had been considered as potential ancestors, many of which may have been close or distant cousins, but Lucy filed all the necessary criteria. If there was ever something that never went extinct but which instead learned to build campfires and work with tools, Lucy was the prime candidate. This has been reinforced time and again since the 1970's.

Meanwhile, the misconception of a missing link has been a die hard fallacy. In the fossil record, there are countless missing links. It should come as no surprise that there are huge gaps all over the fossil record. That is just the nature of the beast. Fossils do not appear everywhere and in the few places on earth that they do, they only appear under ideal conditions. Furthermore, many paleontologists agree that evoluton proceeds in fits and bursts. The opportunity in nature to catch a glimpse of any missing links, may be as unlikely a photographer happening to snap a lightning strike. Even worse because the photographer can prepare for the electrical storm and wait for the moment. A fossil hunter is a wandering nomad with few clues but a keen eye for fossils, When a new species does arise, we have to realize that it does so in an isolated population (not to the whole of the ancestral species). The opportunities to capture that frozen moment in time and location of a speciation event are so rare, that it is surprising that we have as many transitional specimens as we do.

In the hominid branch of evolution, we are especially fortunate and it has been hard to choose which phylogenetic tree is most parsimonious. The problem is not because we are lacking in specimens to fill the so called 'missing links', but rather that we are spoiled with choice. The phylogenetic tree, is abundant with hominid species so that disputes about where to place them revolve more around similarities than differences.

It is so very very important to remember, that Darwinian evolution is a branching phenomenon. A new species does not simply evolve from it's ancestor, one whole species transforming in totality to another, rather, it branches off into a new lineage. A small sub-population begins to exploit a newly discovered and ecologically fertile niche, and that gives rise to a new species. The transition may be short lived and the population that makes the split may be small and isolated from the ancestral group.

The chances that any transitional forms may be caught in the lens of the fossil record are small, yet still we find them. Hominids must have been a diverse group positively bristling with variation. So many must be counted as extinct cousins, (close or distant) that it is hard to know how they all fit. But the idea of a missing link is most definitely an archaic relic of our parochial homo-centric past. There are numerous evidences of the "missing link." However, this discovery leads to more problem. Imagine two cups placed at a distance. It will create one gap. Now place a cup (missing link) at the center of this two cups. Now there are now two gaps created between the now three cups. Once that two gaps again is bridged by more cups, the gaps now are 4, 8, 16 etc. etc. But that gap is now getting smaller and smaller. Sooner or later there will be a continuous train of cups with no gaps.

Answer 2

"The missing link" was a term popular in academic circles during the 1950s, when scientists were looking for proof of human evolution. At that stage, there was only limited archaeological evidence of any species intermediate between modern humans and other primates, apart from the relatively modern Cro-Magnon.
However, numerous primitive human skeletons have since been found in Africa and elsewhere, providing evidence of several intermediate stages of evolution over several million years. As expected, the older the skeleton, the less like modern humans the species appears to be like.
There is no longer a "missing link" as the term was understood fifty years ago.