Embryos have provided evidence for the theory of evolution ever since Darwin published On The Origin of Species in 1859. Embryos provided evidence for (i) transformation of features of the phenotype (jaws, limbs, kidneys) and (ii) transitions between groups of organisms (invertebrates to vertebrates, fish to tetrapods). In the latter role, embryos provided missing links that could not be seen in fossils. Evidence was obtained in two ways: from comparative embryology in laboratory studies and from expeditions to find embryos of organisms thought to represent missing links. I use the four Ps to discuss the search for embryos in evolution.

Life has thrived on this planet for the past 3500+ million years, but the vast majority of this history was conducted by microscopic, largely morphology-less, organisms. All that changed around about 520 Ma with the "Cambrian explosion" of large size, complex morphology, and a fundamental shift in evolutionary tempo and mode. The cause of the Cambrian explosion lies in the unique capacity of motile multicellular heterotrophs (eumetazoans) to drive reciprocal morphological coevolution, leading to novel ecologies, modern-style trophic structures, and a potential for mass extinction. The fossil record provides positive evidence for the absence of eumetazoans prior to 635 Ma.

Virtually all metazoans show signs of bilateral symmetry. Bilaterians likely arose from radially symmetric forms hundreds of millions of years ago. Cnidarians (e.g. corals, sea anemones, "jellyfish") are now recognized as the sister group to the Bilateria and are characterized as diploblastic, radially symmetric animals. However, members of the basal class of Cnidarian, the Anthozoa (e.g., corals and sea anemones), exhibit subtle indications of bilaterality. The starlet sea anemone, Nematostella vectensis possesses: a primary oral / aboral (O/A) axis and a second axis orthogonal to the O/A axis, the directive axis which have been suggested to represent the bilaterian anterior-posterio and dorso-ventral axes. Total genome sequencing of the N. vectensis genome has allowed a more thorough search in silico than previously possible through a conventional degenerate PCR-based approach. We have identified and characterized the developmental expression of orthologs to the key bilaterian patterning genes. Many of these genes are asymmetrically expressed during development in surprising ways that suggest that the antecedents for metazoan body plan organization existed before traditionally appreciated. These data indicate the discrepancy between molecular complexity and body plan complexity and suggests that the cnidarian body plan might be secondarily simplified.

More than forty tyrannosaurid skeletons have been collected in Alberta since 1884. They represent at least four distinct species - Albertosaurus sarcophagus, Daspletosaurus torosus, Gorgosaurus libratus, and Tyrannosaurus rex. Preservational biases favour the preservation of large mature animals, and very young individuals have never been recovered for any of these species. Nevertheless, the discovery in monodominant bonebeds of partial skulls and isolated cranial bones of juvenile tyrannosaurids has revealed some startling ontogenetic changes. Cranial proportions of juveniles are more similar to those of closely related small theropods like dromaeosaurids, whereas the architecture of mature skulls is closer to that of larger more primitive theropods like allosaurids.

The first step in smelling is capture of odor molecules from the surrounding fluid. Various arthropods capture scents using olfactory antennae bearing arrays of chemosensory hairs. We studied the fluid mechanics of arrays of chemosensory hairs on the antennules of a variety of species of lobsters, crabs, and mantis shrimp, and on the antennae of moths. Although the morphologies of these olfactory organs differ from each other, these diverse animals have converged on the same physical mechanism of sniffing (taking discrete odor samples in space and time) by altering the penetration of odor-bearing fluid into their arrays of chemosensory hairs.