Scientists today poured scorn on claims that newly-discovered fossil remains were those of a new species of human ancestor.
They hailed the discovery of the skeletal remains of an infant and adult female, unearthed in a South African quarry, as "truly amazing" - but denied they were a missing link.
They said the bones were an important find but did not represent the discovery of a new species.
One expert said it was "the wrong species, in the wrong place, at the wrong time".
Dr Darren Curnoe, specialist in human evolution at the University of New South Wales, Australia, said: "The discovery of one, let alone two, partial skeletons of the fossil relatives of humans is a rare and truly amazing thing.
"Added to this is the remarkably young geological age of these new finds.
"They are substantially younger than any known member of this primitive human-like group, the australopithecines.
"It also adds yet another branch to our evolutionary tree and confirms an emerging picture of nature's grand experiment with human-like apes, living humans the sole survivor of evolutionary tinkering.
"These remarkable facts would usually be enough to secure researchers a place in the history of science. Unfortunately, they seem not to have been enough in this instance, as the discovery is surrounded by hype and over-interpretation, in terms of its significance.
"To claim that these new fossils represent an ancestor of living humans is misleading and founded in error. Australopithecus sediba is the wrong species, in the wrong place, and at the wrong time. It is way too primitive to be the ancestor of the human genus Homo, one of our direct ancestors.
"For a start, fossil Homo is known from East Africa to be almost half-a-million years older. The skull, tooth and limb bone anatomy of the older Homo also looks very different from those of sediba. Finally, a number of key skulls compared to the new sediba remains have been incorrectly described, leading to false conclusions about its place in human evolution."
Professor Maciej Henneberg, the Wood Jones Professor of Anthropological and Comparative Anatomy at the University of Adelaide, said: "The new find is important.
"Skeletal and dental remains are relatively well preserved and provide large amount of information about the individuals studied.
"Their characteristics fit well into their time period.
"They are a mixture of Australopithecus and human characters indicating a transitional form to Homo.
"Finding of such form is entirely expectable for this time period and the location in Southern Africa. I am not sure, however, whether a designation of a new separate species is necessary.
"In the human lineage there is a natural range of variation of characteristics of individuals and the new finds fit into this range.
"One of the features used by the authors as indicating a new species is the relatively small cranial capacity 420ml.
"Hominid cranial capacity is highly variable, it does not correlate with intelligence and thus some individuals within the same species may have smaller, others larger, cranial capacity. No need to use it as a trait separating species.
"Similar comments apply to other characteristics of the new find." He said the remains were "an important find having transitional characteristics between Australopithecus and Homo, but not necessarily a new species".
Professor Colin Groves, of the School of Archaeology & Anthropology at the Australian National University, said: "We must congratulate the team on a magnificent discovery, while disputing their assessment of its taxonomic affinities.
"The new 'australopithecine' is actually a new species of Homo.
"Judging by the description, it is a South African sister species to the contemporary east African Homo habilis."
He added: "The whole idea of the genus Australopithecus is actually very vague. All too many palaeoanthropologists use it as a place to stick any fossil of the human clade that is not Homo (at the "upper end") or one of the very early ones like Ardipithecus or Orrorin.
"It is what has been called a wastebasket category, whereas if it is to have any value it should be used for a clade within the human clade (a lineage of its own, that is to say): so far, we know only of Australopithecus africanus (a South African species that is somewhat earlier than the new species) that fits this requirement as far as Australopithecus is concerned: all the others generally included in the genus belong to separate lines of descent."
Two papers describing the discovery and dating procedures will be published in the journal Science.