Animation 18.1 The Primary Divisions of Life

INTRODUCTION

Genetic studies indicate that modern life descended from a common ancestor. The last universal common ancestor likely existed more than three billion years ago. Over the course of evolution, it appears that two main lines of descent, drawn as branches on a tree of life, have sprung from the last universal common ancestor: Bacteria and Archaea. A third group—the Eukarya—evolved from within the Archaea.

Video titled: Animation 18.1 The Primary Divisions of Life

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Textbook Reference: Key Concept 19.4 Protists Are Critical Components of Many Ecosystems, p.457

CONCLUSION

In the past few years, scientists have sequenced the entire genomes of many different species from the Bacteria, prokaryotic Archaea, and Eukarya. This plethora of data has allowed scientists to compare how thousands of genes differ or are similar among organisms from these three groups.

Many of the comparisons corroborate the model of evolution that biologists currently hold: that Archaea diverged from the Bacteria long ago and that the Eukarya later evolved from within the Archaea.

Two major endosymbiotic events between the Archaea and the Bacteria resulted in eukaryotic cells having mitochondria and chloroplasts. In addition to these two large tangles in the branches between Archaea and Bacteria, there are many more smaller tangles that show a phenomenon called lateral gene transfer. For example, Archaean species have genes that have been recently derived from Bacteria, and Eukarya also have a number of genes that are of relatively recent Bacterial origin. This DNA sequence data has provided the evidence that lateral gene transfer occurred repeatedly throughout evolution.

For scientists, the lateral transfer of genes has turned lines of descent in the tree of life into a vast and complex network of relationships. Yet it is debatable whether lateral gene transfer has seriously complicated attempts to resolve the tree of prokaryotic life. While it complicates studies in some individual species, it need not present problems at higher levels. Nucleotide sequence comparisons involving entire genomes are revealing a stable core of crucial genes that are uncomplicated by lateral gene transfer. Gene trees based on this stable core more accurately reveal relationships of the organismal phylogeny.

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