Research from Harvard University¹s Museum of Comparative Zoology and from the Cedars Sinai Institute for Spinal Disorders connects several recent fossil discoveries to older fossils finds that have eluded adequate explanation in the past. The report deals with the "homeotic" genetic mechanisms that encode anatomical assembly in the embryo, and their relevance to a series of discoveries of hominoid fossil vertebrae.

The report analyses changes in homeotic embryological assembly of the spine in more than 200 mammalian species across a 250 million year time scale. It identifies a series of modular changes in genetic assembly program that have taken place at the origin point of several major groups of mammals including the newly designated 'hominiform' hominoids that share the modern human body plan.

It concludes that a specific gene change ­ in the Pax system -- that generated the upright bipedal human body form -- may soon be identified. The various upright "hominiform" hominoids appear to share this morphogenetic innovation with modern humans. Homeotics concerns the embryological assembly program for midline repeating structures such as the human vertebral column and the insect body segments.

The critical event involves a dramatic embryological change unique to the human lineage that was not previously understood because the unusual human condition was viewed as "normal."

"From an embryological point of view, what took place is literally breathtaking," says Dr. Aaron Filler, a Harvard-trained evolutionary biologist and a medical director at Cedars Sinai Medical Center's Institute for Spinal Disorders.

In most vertebrates (including most mammals), he explains, the dividing plane between the front (ventral) part of the body and the back (dorsal) part is a "horizontal septum" that runs in front of the spinal canal. This is a fundamental aspect of animal architecture. A bizarre birth defect in what may have been the first direct human ancestor led to the "transposition" of the septum to a position behind the spinal cord in the lumbar region. Oddly enough, this configuration is more typical of invertebrates.

The mechanical effect of the transposition was to make horizontal or quadrupedal stance inefficient. "Any mammal with this set of changes would only be comfortable standing upright. I would envision this malformed young hominiform -- the first true ancestral human -- as standing upright from a young age while its siblings walked around on all fours."

The earliest example of the transformed hominiform type of lumbar spine is found in Morotopithecus bishopi an extinct hominoid species that lived in Uganda more than 21 million years ago. "From a number of points of view," Filler says, "humanity can be redefined as having its origin with Morotopithecus. This greatly demotes the importance of the bipedalism of Australopithecus species such as Lucy (Australopithecus afarensis) since we now know of four upright bipedal species that precede her, found from various time periods on out to Morotopithecus in the Early Miocene."

Citation: Filler AG (2007) Homeotic Evolution in the Mammalia: Diversification of Therian Axial Seriation and the Morphogenetic Basis of Human Origins. PLoS One 2(10): e1019. doi:10.1371/journal.pone.0001019, http://www.plosone.org/doi/pone.0001019

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