Discontinuous morphological changes in the hominid lineage
New data from paleontological discoveries and geochronology show that the pattern of morphological change in the hominid lineage was not linear, but mosaic. Adaptations essential to bipedalism appeared early, but some locomotor features changed much later. Relative to the highly derived postcrania of the earliest hominids, the craniodental complex was quite primitive (i.e., like the reconstructed last common ancestor with the African great apes). The pattern of craniodental change among successively younger species of Hominidae requires, according to evolutionary theories, extensive parallel evolution between at least two lineages in features related to mastication. Relative brain size increased slightly among successively younger species of Australopithecus, expanded significantly with the appearance of Homo, but within early Homo remained at about half the size of Homo sapiens for almost a million years. The fossil record indicates an accumulation of relatively rapid shifts in successive species, and certainly not any kind of progressive changes. (McHenry H.M. Proceedings of the National Academy of Sciences of the United States of America 91 (15): 6780-6, 1994.)
Non-coding regions of the chloroplast genome
The chloroplast genome (cpDNA) of plants has been a focus of research in plant molecular evolution and systematics. Despite a conservative rate of evolution and a relatively stable gene content, comparative molecular analyses reveal complex patterns which are difficult to explain on the basis of mutational changes. There are changes in non-coding regions of cpDNA that are site dependent. Coding genes exhibit different patterns of codon bias that violate the equilibrium assumptions of evolutionary models. Rates of molecular change often vary among plant families and orders in a manner that violates the assumption of a molecular clock. Finally, protein-coding genes exhibit patterns of amino acid change that appear to depend on protein structure, which do not appear to be dependent upon structure/function relationships. (Clegg MT., Gaut BS., Learn G.H. Jr. and Morton BR. Proceedings of the National Academy of Sciences of the United States of America 91 (15): 6795-801, 1994.)
Dorsal thalamus in vertebrates
"The evolution of the dorsal thalamus in various vertebrate lineages of jawed vertebrates has been an enigma." (Butler AB. Brain Research - Brain Research Reviews 19 (1): 29-65, 1994.)
Rapid appearance of Cretacea (whales)
A recent fossil discovery (Pakicetus inachus) in Pakistan has demonstrated the appearance of nearly modern whales (Cretacea) over 50 million years ago. This species had no forearms and a very short femur (with no other hind limb bones). It has high dorsal neural spines and long ventral chevrons, necessary for swimming by dorsoventral oscillation with a heavily muscled caudal fluke. These characteristics necessitate the evolution of a fully aquatic, nearly modern whale from a non-aquatic ancestor in less than 10 million years, which is very unlikely. (Gingerich, PD., Raza, SM., Arif, M, Anwar, M, and Ahou, X. Nature 368: 844-847, 1994.)
Recent appearance of Europeans
Mitochondrial DNA sequences from 74 Swiss individuals were compared to sequences from British and Finish populations. Researchers found that the nucleotide sequence differences between those populations are almost as low as those within the populations. However, larger differences between three African populations, suggest a recent origin of the European populations. Researchers suggested that the low diversity among European mtDNA could coincide with the appearance of anatomically modern humans about 40,000-30,000 years ago or, alternatively, the spread of agriculturalists about 10,000-6,000 years ago. (Pult I, Sajantila A, Simanainen J, Georgiev O, Schaffner W, Paabo S. 1994. Mitochondrial DNA sequences from Switzerland reveal striking homogeneity of European populations. Biol. Chem. Hoppe. Seyler 375: 837-840.)
Back to Recent Problems in Evolution
Last updated 05/29/01