This paper develops a bio-economic Malthusian growth model. By integrating recent research on allometric scaling, energy consumption, and ontogenetic growth we provide a model where subsistence consumption is endogenously linked to body mass and fertility. The theory admits a two-dimensional Malthusian equilibrium characterized by population density and body mass (metabolic rate) of the representative adult. As a result, the analysis allows us to examine the link between, in particular, human biology and long run income, body mass and population size. Off the steady-state we investigate the possibility of cyclical behavior of the size of a population and the size of its representative member. We also demonstrate that a take-off into sustained growth should be associated with increasing income, population size, and body mass. The increase in the latter is, however, bounded and can be viewed as convergence to a biologically determined upper limit.