Our Earliest Animal Ancestors
The Cambrian “explosion” that occurred 543 million years ago is one of the biggest mysteries of biology. It is at this point in the fossil record that a multitude of animal forms suddenly appears, for reasons that are not well understood. The first animals preceded the explosion, but they were presumably small, fragile and ephemeral, containing little or nothing able to fossilize. “The smallest animals … are so tiny that they would not leave any visible record,” says Roger Summons, a professor of earth, atmospheric and planetary sciences at MIT. Furthermore, over a period of more than a half billion years, tectonic forces have broken, twisted, burned and cracked any rocks that might have contained such fossils.
Summons heads a new lead team of the NASA Astrobiology Institute headquartered at MIT, which is looking at the emergence of animal life by examining the biochemistry of microbes; organic and isotopic signs of climate change and evolution; genes essential for animals; genetic sequences that will help elucidate the animal tree of life; and biomarkers dating from before and after the emergence of animal life. These may seem separate topics, but they offer related ways to locate and date the vestiges of early animal life, says Summons.
The earliest fossils of multicellular life date to about 1200 million years ago, but “the sudden appearance of animal life,” some 650 million years later, “at the beginning of the Paleozoic is the one of most enigmatic features in the geological record,” Summons says. The earliest putative animal fossils date to the Ediacarian period, about 630 to 542 million years ago. These primitive organisms “must have had a significant complement of genes inherited from their ancestors, that allowed multicellularity, differentiation and signaling” to emerge, Summons says.
Genetic analysis shows that the first sponges date to 650 to 700 million years ago, says team member Kevin Peterson, an associate professor of biological sciences at Dartmouth College, who adds that sponges were probably the last common ancestor of all animal life. “Our data suggest that animals with guts and nervous systems [eumetazoa], the simplest of which are sea anemones and jellyfish, arose from spongelike ancestors, so to understand the evolution of earth’s biota, we need understand whether this is correct.”
Since about 1996, genetic analyses of ribosomal RNA have shown a close relationship between certain groups of sponges and eumetazoans, which Peterson says is confirmed by analysis of ”housekeeping protein sequences” that are involved in common cellular processes like respiration in much of life. Further study of the housekeeping genes that code for these proteins will help elucidate the animal tree of life, Peterson says. If the lineage from sponges is confirmed, “this would show that the origins of both the gut and nervous system occurred within the context of sponge biology. One of our goals is to try and understand when the genes used during the development of the gut and nervous system evolved, and if present in sponges, how they were eventually wired together to allow for the evolution of these morphological innovations” among animals.
Aerobic respiration and oxygen also figure prominently in the Astrobiology Institute project. Because anaerobic metabolism, which does not use oxygen, cannot supply enough energy for independence and mobility, Summons says the project hopes to get a better reading on oxygen levels in the ancient atmosphere and ocean. “When did oxygen become abundant enough to start aerobic metabolism, so animals could move?”
Because most life lived in the ocean, the extent of ocean oxygenation is especially critical to the rise of multicellular life, Summons says. “Some of us think the ocean was radically different until the appearance of animals, that it had very little oxygen, and the deep ocean was toxic to complex life.”
The project will also try to document the oxygen level in the atmosphere, says Daniel Rothman, professor of earth, atmospheric and planetary sciences at MIT, who is leading a subgroup that will focus on oxygen around 600 million years ago, just before the Cambrian explosion created most of the phyla that are still seen on earth. The analysis will examine the ratio of the carbon isotopes, C-12 and C-13, in sedimentary rocks that formed from detritus that rained to the bottom of the ocean. A higher proportion of the lighter C-12 isotope indicates that the rocks are composed of material that had a biological origin, Rothman says.
Changes over time in the carbon isotope ratio may be more interesting than absolute numbers, Rothman explains. “Generally, when life changes, the carbon cycle [the flow of carbon among oceans, atmosphere, rock, and biosphere] changes,” he says. “It’s reasonable to imagine that as the oceans became populated by multicellular organisms, that gave rise to entirely different food webs,” which in turn changed the composition of the rock-forming organic detritus.
Biological carbon is also tightly linked to atmospheric oxygen, Rothman explains. Finding a higher proportion of organic carbon stored in sediment may signify a higher concentration of oxygen in the atmosphere. “What we are doing, in essence, is looking at how life and the environment co-evolved, and the carbon cycle is where much of the action takes place,” says Rothman.
The new project is based on a renewed recognition of how this planet has been shaped by life. To Rothman, biogeology is a step back to the future. “Geology as a science began in part with the study of the evolution of life, because the widespread appearance and disappearance of specific fossils helped in the characterization of geologic formations.” Long before the age of the earth was even known, geologists recognized that similar fossils often resided in rock formations of similar age, he adds. “Indeed the modern geological timescale is still based on fossils, so biogeology, in a way, is at the origin of geology.”