One of the more contentious issues in the field of biology is the question of whether evolution inevitably generates ever-more-complex organisms over time, and, is so, how to understand that increase in complexity. The evolution of life on Earth from single-celled organisms to primitive multicellular organisms to large-brained mammals appears to suggest that evolution moves in a direction of increasing complexity. But scientists disagree about the extent to which evolutionary history can be adequately described as a progressive, unidirectional movement; indeed, they even disagree about how to measure "complexity," some arguing for a definition based on numbers of different types of cells in an organism, others favoring a measure based on anorganism's capacity to process information. Looking for evidence about the evolution of biological complexity, paleontologist Dan McShea investigated whether the structure of the spinal columns of five groups of animal species became more complex over the past 30 million years. Using such measures of complexity as thickness and length of spines, McShea found indications of increased complexity in some species, but decreases or no significant change in others. Researchers George Boyajjan and Tim Lutz reported similar findings in their study of ammonoids, nautilus-like shelled creatures that existed for 330 million years before becoming extinct along with the dinosaur 65 million year ago. Thespiral-shaped shells of these creatures are constructed from multiple chambers. separated by walls, or septa, the structures of which vary in complexity from one species of ammonoid to another. Although the most complex structures are to be found among the later species, and the simplest among the earliest, there was no steady progression toward increased complexity within any particular lineage. Boyajian and Lutz also looked at the relationship between anatomic longevity was 15 million years, the anatomically more complex species fared neither better nor worse than the anatomically simpler ones did. To biologist Stephen Jay Gould, these results are entirely plausible. Evolution is driven by natural selection, the natural process that results in the survival of individuals and populations best adapted to the conditions of their habitat, he argues, thus perpetuating favorable genetic traits produced by gene mutations. He claims that because it is a local phenomenon, and not a global trend, natural selection favors individuals with traits that are adaptive in particular prevailing conditions – conditions that are just as likely to demand a decrease in complexity as an increase. McShea's findings are congruent with Gould's Point. According to McShea, everyone "knows" that the world of nature is more complex now than it was 550 million years ago. What his results on vertebrate complexity show, he says, is that this global increase in complexity is not expressed consistently in all lineages of animals.