I bring to the environmental studies program an interesting and rare persopective: that of an economist and businessman.(Give me some time and I might even jazz up the HTML ;-)
The first step in evaluating human impact on the environment is to elucidate what we consider human-caused ecological damage. There are, not surprisingly, a great many approaches possibly in defining such a broad concept, but there are a few basic principles which are nearly universal bads. The damages in this illumination are defined with a Darwinian perspective. One such bad is the loss of diversity among non-human population. This loss of diversity can take the form of extinction of plants or animals, a loss of genetic diversity among one species, or a forced-relocation of an existing population. Another bad is the transformation of terra, whether intentional or unintentional, as the result of human activity. Examples are common think 1930 s Oklahoma Dust Bowl and are often caused by agricultural activities. The next step in evaluating the extent human-caused environmental damages lies in the measurement of such damages. Since no dollar value can be placed on the extinction of a species, we must estimate the value that species contributes to the Earth s consumption (evaluating changes in consumption is the only way we can say that one situation is better than another). Needless to say, this is a highly subjective process.
Whether intentionally or not, early humans conducted themselves as a single, integrated aspect of the environment, not users of it. For a species to have a significant impact on the environment in any sense greater than a local one, it must either have numbers large enough to physically damage large segments of other flora/fauna populations or have enough leverage to magnify the effects on the environment of its existing population. Early humans, those living ten thousand years in the past and numbering only around four million, had only the numbers and technology to eliminate through hunting at most around five hundred million creatures per annum. Sustainably maintaining this rate of hunting, would require a population worldwide of roughly thirty billion edible creatures, a very realistic number, especially considering presentday livestock in the US number over one billion. This figure accounts for the effects of other predators on a population. Essentially, early humans simply did not have great enough numbers or great enough technological leverage to eliminate more animals on this planet than could reproduce in a year.
Locally, however, the story was very different. For the earliest time period we are examining, human popoulations were beginning the transition from nomadic, purely hunter-gatherer societies, into grounded, agriculturally based civilizations. As such, they necessarily modified their hunting patterns and methods to center around a single geographical region. A sudden increase in predators in any one region would almost by necessity significantly and negatively affect prey population numbers, leading to occasional extinctions. The best know example, perhaps, is the Pleistocene extinction, a large mamal that disappeared from North American ten thousand years ago. These examples a re rare and often attributed to non-human causes. In this case, prey refers to not only animals but also assorted wild plants useful for their nutritional values. These plants, however, were likely artificially cultivated over the coming years. Even though early humans did not have the opportunity to alter environmental conditions on a global or even regional scale, they nonetheless had a significant local impact.
Fast forward to modern times. The human population is more than fifteen hundred times larger than that of ten thousand years ago. Economic measures of technology have grown by innumerable bounds an estimated six thousand times over the same period. This is an enormous difference in leverage. Consider this example to help put this increase in perspective: for each single spear used to hunt ten thousand years ago, there could now exist about fifteen thousand hunting rifles. In sheer power an ability to cause environmental damage, this is a terrible amplification. Of course, taking into account destruction of animal and plant populations as well as terra for non-nutritional reasons. According to the Union of Concerned Scientists for Environmental Solutions, loss of biological diversity has proceeded at at alarming rate 654 known species over the past four hundred years, far above the historical average. In addition, of all creatures on the endangered species list, less than one percent is threatened because of natural causes. This is not even to mention that 1.5 million hectares of rainforest are destroyed annually.
Without argument, the environmental damage done as a result of human activity in the last hundred years far exceeds any damage done over the prior 3.5 million. This increase in damage is the result of not only increases in population, but the leveraging of technologies and tools designed to improve human consumption. Barring a change of attitude, however, this increase in level of damage was unavoidable as its root cause is the natural desire and perhaps even base purpose of humanity. Considering the complex relationships and interdependencies present in the natural world, we well know that there exists a point of no return. After environmental destruction proceeds past this point, there is no turning back Earth will be destined over hundreds and perhaps thousands of years to repeat the fate of Easter Island. The fortunate news is that, odds are, this event horizon is most likely a long way off. The future is always longer than the present. At the same time, the imminent possibility exists that that point has long passed.
 Estimates calculated according to numbers provided by the Open Door Project