Recall that the eukaryotic cell plan is needlessly complex.
Recall the evidence suggests this needless complexity was essential for the emergence of metazoan-type existence.
Of course, the nucleus, even without the chromosomes within, is a very complex and sophisticated structure. Yet just how old is this complexity?
First, consider the fact that the nucleus contains multiple portals known as the nuclear pore complex. This complex regulates the traffic of proteins and RNA across the nuclear envelope.
It is probably the most complex protein structure inside the cell:
It turns out that the last common ancestor of all eukaryotes (the LECA) had a nuclear pore complex that was essentially the same as that which exists today:
Overall these data provide conclusive evidence that the majority of NPC architecture is indeed conserved throughout the Eukaryota and was already established in the last common eukaryotic ancestor. These findings strongly support the hypothesis that NPCs share a common ancestry with vesicle coating complexes and that both were established very early in eukaryotic evolution. 
These results significantly extend earlier results and, importantly, unambiguously place a fully-fledged NPC in LECA….. Our results indicate that all major protein subcomplexes in the Nuclear Pore Complex are traceable to the Last Eukaryotic Common Ancestor (LECA). In contrast to previous screens, we demonstrate that our conclusions hold regardless of the position of the root of the eukaryote tree. 
And then there are the karyopherins, a class of proteins that functions to shuttle material in and out of the nucleus via the nuclear pore complex. Again, the last eukaryotic common ancestor appears to have a modern-like level of complexity with regard to these players:
At least fifteen KAP-β subfamilies were established early in eukaryote evolution and likely before the LECA. In addition we identified expansions at multiple stages within eukaryote evolution, including a multicellular plant-specific KAP-β, together with frequent secondary losses. Taken with evidence for early establishment of NPC architecture, these data demonstrate that multiple pathways for nucleocytoplasmic transport were established prior to the radiation of modern eukaryotes but that selective pressure continues to sculpt the KAP-β family. 
So the nuclear pore complex and main mechanism for transport through it have existed much as is since the last ancestor of all eukaryotes. But there is more. The nucleus is part of the endomembrane system:
The endomembrane system is composed of the different membranes that are suspended in the cytoplasm within a eukaryotic cell. These membranes divide the cell into functional and structural compartments, or organelles. In eukaryotes the organelles of the endomembrane system include: the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vacuoles, vesicles, and the cell membrane. The system is defined more accurately as the set of membranes that form a single functional and developmental unit, either being connected together directly, or exchanging material through vesicle transport.
So did LECA contain such an elaborate arrangement?
The eukaryotic endomembrane system is responsible for the biosynthesis and transport of proteins and lipids, and for the definition of the major subcellular compartments. Recent work indicates that the endomembrane system is ancient, with near modern complexity predating the radiation of the major eukaryotic lineages. 
And what about the protein machinery that controls the flow through the endomembrane system?
In membrane trafficking, the mechanisms ensuring vesicle fusion specificity remain to be fully elucidated. Early models proposed that specificity was encoded entirely by SNARE proteins; more recent models include contributions from Rab proteins, Syntaxin-binding (SM) proteins and tethering factors. Most information on membrane trafficking derives from an evolutionarily narrow sampling of model organisms. However, considering factors from a wider diversity of eukaryotes can provide both functional information on core systems and insight into the evolutionary history of the trafficking machinery……. These data further support a highly complex LCEA and indicate that the basic architecture of the trafficking system is remarkably conserved and ancient, with the SM proteins and tethering factors having originated very early in eukaryotic evolution. 
So there you have it. The nucleus, the nuclear pore complex, the endomembrane system and its control components, were all present in the last ancestor of all eukaryotes. To this we can also add the mitochondrion and the flagellum/intraflagellar transport, which were also present. What this means is that a modern-like eukaryotic cell plan was in existence at the very base of the eukaryotic tree and could thus work to nudge and channel subsequent eukaryotic evolution to facilitate the emergence of metazoa.
1.DeGrasse JA, DuBois KN, Devos D, Siegel TN, Sali A, Field MC, Rout MP, Chait BT. 2009. Evidence for a shared nuclear pore complex architecture that is conserved from the last common eukaryotic ancestor. Mol Cell Proteomics. 8(9):2119-30.
2.Neumann N, Lundin D, Poole AM. 2010. Comparative genomic evidence for a complete nuclear pore complex in the last eukaryotic common ancestor. PLoS One. 5(10):e13241.
3. O’Reilly AJ, Dacks JB, Field MC. 2011. Evolution of the Karyopherin-β Family of Nucleocytoplasmic Transport Factors; Ancient Origins and Continued Specialization. PLoS One. 6(4):e19308.
4. Field MC, Dacks JB. 2009. First and last ancestors: reconstructing evolution of the endomembrane system with ESCRTs, vesicle coat proteins, and nuclear pore complexes. Curr Opin Cell Biol. 21(1):4-13.
5. Koumandou VL, Dacks JB, Coulson RM, Field MC. 2007. Control systems for membrane fusion in the ancestral eukaryote; evolution of tethering complexes and SM proteins. BMC Evol Biol. 7:29.