Hi, need to submit a 1250 words paper on the topic Evolution of Planktonic Larval Forms from Benthic Bilateral Animals. Some scientists argue that the pelagic larvae existed first and these evolved to form the benthic adults. On the other hand, other scientists argue that benthic adults existed first and slowly evolved into the pelagic larvae. Both are but hypotheses that have their limitations. Scientists from both sides have supported the theories and provided possible explanations of how evolution took place. The hypothesis suggesting that the benthic metazoans were the original and then the pelagic larvae evolved later seems more logical. According to this theory, the first bilaterians consisted of triploblastic animals that exhibited bilateral symmetry.

Phylogeneticists have discovered that the most basal bilaterians are acoels (Raff 1474). This serves as evidence that direct development probably occurred because they contain posterior, interior, and middle Hox genes. These genes are responsible for direct development that took place. Other bilaterians displaying more complexity than acoels could transcribe genes responsible for the development of eyes, mesoderm, nephridia, and heart. The presence of such machinery proves that the development of bilateral body features occurred in steps. This makes it possible for one to visualize how the process of larval evolution occurred. Subsequent steps in the evolution produced a different generation of phyla of these organisms. The planktotrophic larvae developed indirectly producing the intercalation of larval origins. The intercalation of larval origins occurred during the Cambrian period that brought along radical changes. The Cambrian period resulted in the formation of diverse phyla of these organisms. The diversity is a reality on observation of the modern phyla. Evolution resulted in basal clades that define the precursors of modern phyla. The evolution of planktonic larvae was a secondary pathway. The larvae-first theory asserts that larvae existed in the Precambrian era (Nielson 208). However, studies defy this claim.

This theory is self-supporting because an understanding of the key features exhibited by larvae gives the theory logic. Since the structure and adaptations of the larvae are minimal compared to adults, it is possible that the co-option of developmental genes in the adult resulted in the secondary development of larvae.