How big can animals get on a given planet? Michael Denton has made a compelling case that given a habitable planet the size of Earth, humans must be the right size to have the ability to use fire and create technology (The Miracle of Man, pp. 194-197). Humans appear to have the optimal size for these purposes, with arm lengths and hand shapes appropriate for swinging axes and hauling wood for the controlled use of fire.
These optima have allowed humans to develop modern technology. Physical laws, he shows, rule out the possibility that we could do all we do if we were as small as ants or as tall as 60-foot giants. “We could be neither fire makers nor metallurgists if we were significantly smaller” than we are, he says.
On the other hand, it is fortunate that the ability to hew wood and mine for ores does not necessitate kinetic forces much greater than those that can be generated by organisms of our dimensions. While significantly larger android beings could exert greater kinetic forces, the design of a bipedal primate of, say, twice our height would be severely constrained by kinetic and gravitational forces and be structurally problematic.
Why? For one, and as discussed in Chapter 9, mass (and weight) increases by the length cubed (L3) while the strength of bone and the power of muscles increases only by the length squared (L2). [Emphasis added.]
If we were giants, we would be at daily risk of shattering our bones into pieces. And that’s not all that physics requires of us. In addition to the right body size and shape, Denton argues that we must live on the right size planet. If Earth were smaller, ants might do fine, but such a planet could not maintain a life-giving atmosphere. If Earth were larger, gravity would “exacerbate the dangers of tripping” for upright bipedal creatures like us. Higher gravity would impose “severe constraints on the capacity of muscles to empower movement and support an upright bipedal stance.” Undoubtedly those constraints would impose ripple effects on all our systems: circulatory, respiratory, metabolic, and everything else.
Design Inference from Bones
What about size limits on big animals, like the giant sauropods? Given Earth’s gravity, are there physical limits to their existence, too?
We don’t expect to find sauropod technology, but the same physical laws (mass increase with height) apply to them. The giant sauropods like Titanosaurus, Ultrasaurus, and Seismosaurus may have lived close to the physical limits of size on an Earthlike planet. Blue whales grow even more massive but are buoyed by water. Extant whales and large land mammals allow us to test theories about design limitations. What about giant creatures that can only be known from their bones?
Denton says that large land animals reduce the risk of falling (and breaking their bones) by walking on all fours. Even so, falling endangers horses and cows. A two-meter tall man faces 20 to 100 times the force of a fall compared to a two-foot child, he notes. (p. 196) The larger the animal, the greater the danger. The fact that giraffes and elephants can survive to adulthood suggests that their large bodies are well engineered for stability. From reconstructions of dinosaurs, the bipedal ones (like theropods) appeared to have good balance because of their tails, and the quadrupeds also had long tails and big feet for stability. In every case, engineering for stability would require attention to bone density, muscle strength, and kinesthetic sense (e.g., inner ear balance organs).
Scientists at the University of Queensland in Australia applied engineering design principles to the largest land animals that ever lived: the sauropods. They applied what is known about tissue anatomy and physiology of elephants and other extant large animals. Elephants, despite appearing to walk flat-footed, actually walk on their toes. Their heels are cushioned by soft tissue padding made of muscles, tendons, ligaments, cartilage, and sole skin. This padding is a design requirement due to the elephant’s high mass. The Australian team wondered if dinosaurs also required foot pads.
Without having living sauropods available to observe, physiologists can infer things from the data available. Knowing that mass increases by the cube of length, biophysicists can estimate requirements for bone density and muscle strength for the giant dinosaurs. The fossilized bones, though mineralized, provide cross-checks for those inferences to a certain extent. Trackways also inform the diameters and shapes of dinosaur feet. What can’t be checked is the soft tissue padding around the foot bones, which is not preserved in fossils.
The Queensland team, led by Andréas Jannel, inferred that a sauropod would have needed padding in its feet to survive. And since dinosaur foot bones are not homologous to elephant feet, the padding had to be designed differently than the padding observed in the hind foot (pes) of an elephant or rhino. Their open-access paper in Science Advances explains how they inferred foot padding as a requirement.
Our results show that, irrespective of skeletal pedal posture (Fig. 1), all sauropodomorph specimens examined (i.e., representatives of distinct clades and diverse body sizes) would have been unable to support their weight without a soft tissue pad in the pes (Figs. 2 and 3). All skeletal morphotypes without a soft tissue pad resulted in maximum von Mises stresses higher than 500 MPa [megapascals] for all pedal models (up to 5000 MPa as recorded in Rhoetosaurus brownei; Fig. 3). As expected, bone stress increased principally in the shafts of each metatarsal and the most proximal phalanges (Figs. 2 and 3), likely due to the pedal posture, boundary conditions, and the fact that the metatarsals are the longest bones in the sauropod pes. Mechanically, it is highly unlikely that sauropod pedal bones could have withstood bone stresses of this magnitude without failing. This is because sauropod bones have been shown to retain the general structural properties of Haversian bone tissue seen in modern birds and large mammals, indicating that they were most likely subjected to comparable mechanical constraints. In humans and bovids, cortical bone (e.g., such as in the femur) has been evaluated to withstand maximum stress < 150 to 200 MPa (44, 45). Hence, within the context of comparable loading regimes, the mechanical state of each sauropod model examined suggests that all skeletal pedal postures would most likely have resulted in mechanical failure (e.g., stress fractures). This state would have been intensified when subjected to repetitive heavy loadings, as would be expected during normal locomotion, ultimately resulting in fatigue fracture in all digits. Being unable to support or move properly, the high probability of mechanical failure would have had a substantial impact on the animal’s survival.
Thinking Like Engineers
The team used this reasoning to construct models showing where soft tissue would have been needed for cushioning the foot bones of various sauropod species. That’s thinking like engineers. These scientists, being Darwinists, believe that the engineering was supplied by natural selection. But that’s a story for another time.
The fact remains that they made a design inference based on physics, fossils, and comparisons with living animals. As Denton argues in his “Privileged Species” books and videos, physical laws constrain what beings are possible on a habitable planet. That humans and other living beings thrive so well, and have thrived through Earth’s history, suggests that prior fitness was designed in the fabric of the earth and the universe. And if that is the case, then it’s not a big leap to reason that the specific forms these organisms took were also crafted according to engineering principles — with some artistry thrown in, too.