Supporters of evolution believe that a common genetic heritage is shared by all currently living organisms on Earth. Modern evolutionary biology suggests that all organisms came from the last universal common ancestor of all life on our planet. However, evidence about life's single ancestry is needed to support this claim.

There are five major lineages of animals that arose early in evolution and survived until the present day. These include sponges, ctenophores, placozoans, cnidarians, and bilaterians. Morphological studies exclude the groups of sponges and ctenophores, and for many years, experts have been investigating two alternative hypotheses connecting their relationship with other animals. The competing sponge-sister and ctenophore-sister hypotheses challenged scientists as they tried to answer which lineage split first among the animals.

Connecting the lineage of sponges, ctenophores, and other animals is difficult to resolve, and it poses a challenge in studying animal evolution. Using the pattern of chromosomal gene linkage without considering gene order has become an important consideration in resolving the ctenophore-sister versus sponge-sister argument.

The Search for the Sister Group

A group of researchers from UC Santa Cruz, the University of Vienna, and the Monterey Bay Aquarium Research Institute tried to answer the mystery of the critical moment when an ancient organism split off into diverse species of animals today. The team employed new techniques in comparing the genomes of ctenophores and sponges to look for evidence of the earliest moments in the evolution of animals.

Also known as comb jellies, ctenophores are marine invertebrate organisms found in oceans around the globe. Although they are transparent, they produce bright, iridescent colors. Marine sponges, on the other hand, are sessile invertebrates that can thrive in temperate, polar, and tropical waters. They are multicellular animals with bodies covered with pores.

Traditionally, experts compared the arrangement of genes on chromosomes, but they realized that this method could not give reliable answers about an animal's evolutionary history. To solve this problem, the team conducted complete genome sequences to observe the events that happened at the chromosome level which were previously unseen.

DNA samples of comb jellies were taken from Monterey Bay for a sequencing procedure. Assembly efforts were made in the biomolecular engineering laboratory of UCSC, where the entire length of each chromosome for two comb jellies and two marine sponges was analyzed. Upon obtaining the complete sequence, the scientists searched for linked gene patterns to determine if sponges or ctenophores are the closest relatives of all other animals.

After examining the genome of comb jellies at the chromosome scale, scientists observed a group of genes with patterns very different from those of other animals. Another discovery includes the pattern of genes shared between comb jellies and non-animal species such as filasterean, ichthyosporean, and choanoflagellate. These shared patterns of genes have been rearranged and mixed in all other animals. It only shows that comb jellies are divided first before all other animals.

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Resolving the Mystery of Gene Linkage

Identifying the first diverging animal lineage provides a way of understanding the evolution of basic animal characteristics. Proving the role of comb jellies as being distantly related to all other animals means that they could be the earliest lineage in the Tree of Life or the branch where all the other animals came from before evolving separately.

Resolving the question of gene linkage will also help scientists understand the formation of anatomical systems in the earliest animals. It can also explain how evolution happened from single-cell to multicellular organisms.

 

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